In a bot is well programmed, there is no easy and fast way to spot it. As a HUD-user you should look for players, who are very active in your game, so that you collect a large sample on them quickly, and where their stats are almost to perfect.
Personally I have seen one account on
pokerstars, which I suspect was a bot. The reason is, the account played a lot, and the stats, that developed on my HUD were unusual and would require a lot of discipline to maintain, if a human was playing like this.
In short the player was aggressive but it also seriously overfolded to aggression. Which is kind of exactly, how you might want to set up a bot for playing in the micros, because it keep it out of trouble, and people in the micros tend to be under
bluffing in most situations.
The account was playing a time limited game called "show time", where all cards are folded face up. Using a bot for this kind of game makes sense, because its easier for a computer to digest and fully take advantage of information about dead cards.
Now I dont know, if this account was a bot, and I never reported my suspicion to PokerStars, because it was so loosely founded. Also the game stopped after a few month, and I never saw the account on regular tables.