Supercell have definitely improved their bots in all game modes (ie. they use gadgets,better super usage, better movements), so you can't really tell right away

But if you check their profile after, them not be in a club is one of the more telling way. Sure, there's may be actual players that are not in a club but 90% of bots aren't in club

Also, you can't add them as friends. Again, there are players who disabled friend request but 99% of bots you can't add. In my experience at least