Part One: Accidental or Intentional?
The first situations we will look at could happen accidentally to anyone while playing a friendly game of euchre. Unfortunately, these same "accidents" can be made to happen intentionally. This fact alone serves the unscrupulous player as an easy way to try to gain the advantage over an opponent while leaving open the claim that the incident had simply occurred unintentionally.
A. The Renege
1. Illegal Ruffing - This is perhaps the most common way to cheat at euchre. The lead player throws an off-suit and the opponent lays trump on it even though he is supposed to follow suit. The off-suit will be thrown in on a later trick in an attempt to go unnoticed. A person doing this will often mix the up the cards in the tricks they take to make it more difficult to trace in case the opponents  catch them at it and try to back-track the hand.
2. Trump Holdout - Trump is led and a player holding a lower trump plays an off-suit instead of following correctly. On a later trick, much to the surprise of the making team, the same player trumps an off-suit lead. This is a very amateur move because even mediocre euchre players usually notice when an opponent is void in trump. Throwing off a card of the same color does make it a little less noticeable though, and holding out the left bower makes explaining an "accident" slightly more believable since it is such a common beginner mistake.
1. Special attention should be paid to players tricks that have not followed suit.

2. Players should demand that their opponents stack their tricks in neatly staggered piles.

3. Rules should be in place penalizing guilty parties at least 2 points for reneging (4 during loners) no matter accidental or not.
B. Bogus Scorekeeping
1. High Mark-up - Perhaps the easiest way to score points is to simply mark the point up! Anyone who doesn't believe that the players in their friendly game would ever do this may be fooling themselves. I have often found that even honest players sometimes can't resist the temptation to mark 2 points for 4 tricks taken (especially if they took all but the first one), of course "accidentally". "Oh, you guys got the first trick? I'm sorry, I thought we took 'em all."  Sound familiar?
2. Roll-back - Whenever two playing cards are used to keep score (probably 95% of the time) all sorts of "accidents" can happen. A little bump here, a sneeze there, even a blatant hand reaching over and unmarking a point or two. It happens all the time, believe it! Remember, no matter what the game is being played for, it is still a contest. People become competitive and sometimes can not stop themselves from cheating. Add money to the mix and the incentive only increases.
1. Mentally keep track of the score at all times. In the event that the scorecards are knocked over, you should know how to reset them.

2. For serious games, use pegboard style scorekeepers or write it down on paper.
C. Information Sharing
1. Crossboarding - In friendly games, players often talk a lot. Sometimes the talk can be interpreted as "illegal" as it pertains to somebody's current hand. "Wrong color!" "Are you going alone?" "Damn, I'm bleeding' again!"  These phrases may or may not be true and can slide in a friendly game. In a money game however, this form of "bull" talk will tend to cause problems, especially if the statements are accurate. The accused player will often claim that the talk was in jest and not meant to represent the hand.
2. Signals - Rarely is someone suspected of signaling if they were in fact unintentionally making a motion that looked like something else. For example, clutching or holding chest (hearts), scooping or digging (spades), grasping ring finger (diamonds), hammering action (clubs) are all obvious signals that people who have never even met before could use to let their partner know what suit they want to call. Organized signaling can be a lot more cunning. Little movements with ashtrays, subtle facial expressions, winks, nods etc. can be worked out and practiced by a partnership who wants to tell each other exactly what each other holds. The accused player will claim that the motion was coincidental and meaningless.
3. Exposing Cards - If the other team is not paying close attention to the game, exposing techniques can be employed. Some of these include;
a.) Misdirection- If the players are relaxed and there is a lot of conversation going on, a partnership simply has to take turns telling loud descriptive (usually funny) stories complete with hand gestures to capture the full attention of the opponents. When all eyes are fixed on the story teller, his partner very quickly, yet smoothly turns his hand around, exposing it to the speaker. (below)
While Eric (L) reveals the conclusion of his story, his partner reveals his entire hand!
b.) Flash dealing - This is often done by inexperienced card players or card sharps trying to imitate them. As they go to pitch a card to the opponents during their deal, their deckholding thumb sloppily pushes the next card (going to anyone other than the dealers partner) forward and briefly lifts the edge of the card, exposing the corner of it to the dealer's partner. When this unintentional "accident" is purposely recreated, the dealer's partner only glances at it the second that it is easily visible so as to not draw attention to the fact that he is looking at the cards that the other players have before they get them. Oops!
c.) Glims - Glims are shiny devices that are set inconspicuously in front of the dealer that reflect the face of the cards up to him as they are being dealt. This enables the dealer to see every card just before they go into the players' hands. The dealer can even spread the kitty over the glim as if to count the cards and obtain knowledge of them also. Of course the glim has to be something that doesn't look out of place at the card table. The most common glims used in euchre games are those shiny Zippo style cigarette lighters. Poker players also like to use money clips. There doesn't appear to be anything out of the ordinary during a euchre game if a player has a pack of smokes and a lighter out on the table in front of him. Some euchre cheats only smoke during euchre games (or at least bring cigarettes and a shiny lighter with them). Of course, if suspected, the dealer doesn't realize that this reflective device was positioned below the cards and will claim that he never looked down and saw anything unusual.
d.) Wrong Lead - A very common mistake for many euchre players to make is to lead out of turn when their partner actually took the last trick. If it is pointed out by the opponents, it is usually politely withdrawn, yet the partner of the guilty party obtains valuable knowledge that could influence his next lead.
e.) Fumble - Another "accident" that happens from time to time is a card simply falls from a player's hand and lands face up. In this case, the player will usually offer an excuse and expect to be allowed to pull the card back and act like it never happened. Cheaters will try to imitate this "accident" to show his partner a vital card in his hand with the expectation of being allowed to get away with it.
1. Euchre players should watch the player to their right during the entire game. When both players do this, they are always facing the action and preventing either opponents from signaling or exposing cards at the same time.

2. There should be nothing on the table during a money game other than cards and score markers.

3. Rules for money games should forbid excessive table talk, and definitely not in any language other than English.

4. Dealers that can't keep the deck down should be made to redeal the hand and given a warning that future exposed cards will cost them 2 points and the deal. Keep in mind that professional card "mechanics" (master manipulators) will attempt to imitate an unskilled card dealer to draw attention away from themselves.
The Total Bluff - The thing about this maneuver is that it should never be performed successfully, but I've seen it work quite often!  Many times two players are in on this together but they don't have to be. What basically happens is a trick is taken by a losing card. This of course sounds absurd, but it does happen and can often be explained as an accident. When a couple of players are particularly unscrupulous as partners, they may attempt to get away with it regularly. It works best when the game is going fast and a weak bid is made on the second time around.. Many times the opponents' low off-suit will be taken by another even lower card that is the same color as the trump suit. Both partners will react exactly as if they had thrown trump and one of them will quickly scoop up the trick, sometimes pushing it to their partner, while the one who played it leads. This move of course requires that the opponents are not paying close attention to the game.
Players must pay close attention to all cards played.
"I've seen many euchre players do this unintentionally(!). If a partnership intentionally does this, it'll easily destroy opponents who don't pay attention.
Also: This guy needs to be told to "keep 'em down". Next violation, he loses game!