Before I answer your question, I'm curious as to what you're getting at by asking this? I mean, I don't know how experienced you are, but by saying someone is town because 'they wouldn't play like that if they were scum' is so inherently faulty I don't know what to say; any Mafia player worth their salt should be able to fake town if they're scum.
As for how I personally scum hunt, it depends on the situation. However, the first thing I start to explore is how real their contributions are to the game. Is there substance someone's posts/reactions or are they being faked in order to look productive. The biggest thing scum have to do when they play is to blend in with town, which means giving reads and justifying mislynching townies constantly (unless scum bus, but you might clear that person because why on earth would scum do that?). From there, you can branch out in different ways. When it comes to reads, are they consistent, to they make sense, do they have evidence and a basis to exist on. When voting, where is that vote coming from, is it sheeped, does it feel authentic, is it an easy lynch. To sum it up, there are lots of ways you can scum hunt, but I wouldn't say one method is to clear someone by simply saying 'nah, scum wouldn't do that'.