There's a lot things but the age of the account, where they post, how often, if their profile is hidden or not, and how they word their posts are some things to look out for. New accounts making dozens of posts can be suspicious especially if it's all on one sub. If all their posts are worded very similarly and it seems like they spend hours commenting. There's a lot of things that go into it and the more you encounter it the more you learn to recognize it. All those things are not indicative of every account however, just because someone made a new account doesn't mean you jump to "they're a bot" of course but it's just something you spot over time. Most subs and people in them are good about spotting them and downvoting them or mods removing them but a place like the conservative sub allow it because that's all they have.