This is a quick snippet from my book 'The Book of Roleplaying Hints, Tips and Ideas', available from DrivethruRPG and on Kindle (see the right hand sidebar).
Roleplaying games have a set of rules to adjudicate actions and abilities and these are reflected, in most cases, in the use of dice. So why do some roleplayers feel it necessary to cheat? The idea of a high adventure game is to inject a little of the chance and danger inherent in such things. If a bad roll is made, it does not reflect badly on the player, it's just the way things turned out and it's a sign of good roleplaying to take the rough with the smooth.
There are five general types of cheaters:
1) The "Pooper Scooper" who will roll their dice and pick them up straight away before anyone else has a chance to see the result and claim they succeeded.
2) The "Ready-To-Rumble Roller" who will claim they succeeded with the dice that are already lying on decent numbers on the table, which were not actually rolled.
3) The "Bombardier" who will roll their dice one at a time, and every time a low dice comes up they will slam their next roll into the previous dice in the hope of knocking it onto a better number.
4) The "Houdini Skills" players who suddenly acquire a skill or increased ability to help them out of a situation, usually added to the character sheet secretly during play.
5) The "Phantom Equipment" player who will suddenly have an item or tool appear on their character sheet, again added during play.
There is no sure way to guard against these cheaters, especially in large group games where there is a lot to be aware of. There are some precautions you can take, however. Make sure that, before play starts, the group is aware that all rolls are to made in the open and watched by others. (The GM may be exempt from this, depending on their use of GM screens and wanting to have the chance to have more control over the game). Then the player/GM has no choice but to make the roll. Also, rolls must be made with all the required dice thrown at the same time. This way, the group is aware that rolls are being monitored and pre-warning them means that players don't feel picked on.
Don't worry too much about weighted dice. These little monsters are easy to spot as they don't roll naturally and have a tendency to spin when landing on their set number. You can check most of the dice before play, anyway. Have photocopies of the PC character sheets to hand to the GM, and make sure as a player that you've had a good look over other player's sheets (group style/policy permitting). This way you'll have an idea what each player is capable of and what they own, and have an insight into the possibility of cheating.
Case: During a strange game of Call of Cthulhu, the group was skulking about a sunken church in the Black Forest of the Rhine when they were suddenly attacked by ghouls. Single handed, one of the weakest characters in the group managed to hold off the ghouls with a machete and pistol while the others grabbed artefacts and made a run for it. He was hailed the hero of the encounter...until it was realised that no-one had actually seen any of the rolls made, and that the items "pistol" and "machete" were not actually on the player's character sheet equipment list.