The standard builtin AI mplayer does not cheat; it always plays
according to the same rules as you do. This should be true of any AI in
Xconq. If you have evidence that would seem to indicate that any AI
is using information it should not have, or is otherwise cheating, that
is a bug and should be reported. Note that sometimes the AI is smarter
than you are, or moves more quickly; it may very well have spotted one
of your units and disappeared again without you noticing.
Cheating by human players is possible, though not easy. For instance, a player could examine a saved game and learn all kinds of things, perhaps even starting up a separate game from it. As always, human ingenuity will defeat any purely technical defenses, and the best way to forestall cheaters is to refuse to play with them. If you suspect cheating, look at the game history and the final game statistics for things that seem suspicious.