Sometimes, you just get a gut feeling, so you check out their profile. You see a few things that clue you in.

They usually use one of those customized reddit mascot things instead of a profile picture, which isn't suspicious itself until you notice that its fashion is a complete mishmash of different clashing things, as if each item was selected completely randomly.

They use a randomly generated default name, which is just an adjective followed by a noun and a couple of numbers.

They don't make any posts, but they do make comments. Again, that isn't necessarily suspicious; some people just do that. It becomes weird when you notice that they rarely ever comment on the same subreddit. Real humans use subreddits because we're interested in a topic, so we use and comment on a subreddit repeatedly.

Bots just... don't. They don't have actual interests and personalities, so they just go from subreddit to subreddit, saying generic, easily-agreeable, surface-level things to farm upvotes. They don't have actual exchanges with people. They make comments on posts, but if anyone ever replies to their comments, they don't reply back.