I know of no real way to detect this. If such a thing exists for JA (I've heard rumors) it must be very rare.
Most people will tell you that aimbots in this game really wouldn't make much of a difference (since most of the shots don't hit instantly, move slowly and can be dodged, blocked or pushed away).
About the only place it would be a real problem would be in a mod like "Instagib." For example bots in that game (if given perfect aim) become a MAJOR PAIN, at least according to Amidala who likes to run that mod, and I believe it! ; p
About the only thing for now would be to ask the person, and record a demo to look at later. It would be very hard to tell, and I wouldn't jump to conclusions assuming people have them and throwing accusations around. It'll just make you look like a sore loser.
Now if the person admits they have one, well that's something else entirely...
Still, calling somebody a cheater is never a good thing to do, unless you have definate proof.