The wargaming community is a very small community.
Good friends coming together to move around some miniatures, roll some dice, and blow things up.
Even on the tournament scene people tend to know each other.
Cheating is rare for the most part, and reputation is everything.
But when it does happen, what are some of the ways it goes down?
I should first say that there is a difference in getting a rule wrong, or interpreting things differently which might feel like cheating, vs. deliberate intentional cheating- often with the dice.
Loaded dice are often the first question that comes to mind, and I'd have to say these are rare. I've never seen them, and only know of one case in my local meta of using loaded dice in 10+ as a joke on a friend who we all like to agitate.
Loaded dice often look funny, roll funny, and stand out.
But what about cooked dice?
Chessex dice are known for this since they come in cubes and packs with dozens of the same dice...
Take a few and put them with the "6" facing up- zap them in the microwave for a bit.
Zap. Cool. Repeat.
This cooks the weight and pulls it down so the "6"'s want to land face up.
In a tournament scene, one of the easiest ways to prevent this and other physical dice manipulations is to issue out a dice cube or set of dice to everybody and have that be part of the registration fee.
Everybody using the same dice, no exceptions.
Other ways, which are harder to detect, as they use misdirection involve using the dice pools that you roll- we see this in games that use lots of dice like Warhammer 40K.
If I have to roll twenty or so dice to see if my attack hits, count out twenty, but add an extra die or two, roll them and quickly take off this misses.
This crude sleight of hand is the method often employed to get to roll more dice and increase the odds of a successful hit, or saves, etc.