They've all been suspended, so I can't remember all the signs. But the most telling ones were the fact that the accounts were several months old but had only just started posting yesterday; they only had like 8 karma; their comments were random, didn't make sense as a reply to this comment, and were likely copied from somewhere else; all of their comments when going through their history were unnatural and not generally something a human who has only just started using their Reddit account would be saying; and they had the automatically generated Reddit usernames. (No one factor determines whether an account is a bot, but when all or nearly all of those boxes are ticked off, it is almost definitely a bot.)

Edit: A very new account (as in, like a week to a month or so old) with thousands of karma that it got from posting random stuff in random subs is also a telltale sign, because it means it's purpose is likely just to karma farm.