There are ways to cheat that are impossible to detect. At least one that I know of anyway. The only way to combat it is to play well known opponents AND keep your own nose clean and honorable.
If you think your opponent is cheating, ask a veteran before sending out accusations. A few times I have been accused of cheating because what I was doing was misunderstood and considered impossible (7 or 8 phases in a turn for example).
Be a gracious winner. Someone who takes the time to cheat will probably feel good about themselves and enjoy rubbing it in.
A cheater probably won't engage you in helpful dialogue either.
A cheater PROBABLY won't have short turns. Every turn will be long and extremely good. This can be especially curious if the side they are playing has poor overall proficiency.
Keep your own nose clean. People DO ask other players what they think of so-and-so (ie YOU!). If you use an undo let your opponent know why. I prefer to let undos go unused as part of the penalty to myself for not paying attention. I think in 18 months I have used around 5 undos. I have seen other players have 5 undos in a turn! (none of my opponents - someone else's).
In summary most of us play this game because we love the challenge and enjoy both sides of the win/loss column as long as all games are fairly fought. I suppose cheating could be a problem, but my instinct tells me the cheaters don't really enjoy the game enough anyway and probably drop out of sight after a few games.