Well for botters that are farming loot and stuff, it is quite easy to tell. First just watch how they walk, are their movements rigid and unhuman like? Thats one way to tell, also just watch where they go, do they follow the same exact path and go to the same exact points. Also, send them a whisper, and ask them a question or two, to make sure that they aren't really using a afk whisper bot. You can also hit the mob that they are going for, and if they keep killing it, thats another indication. You can really exploit them by hitting their mobs first and then looting them.
There are many ways to know, just observe them for 15 minutes or so, and you'll know.