I haven't said a whole lot thusfar, so let me at least take the time to mention what I look for in scum:
They have very little incentive to lie. Unless they come under extreme pressure, they're likely to play very similar to town. Everybody is going to start getting vibes from everyone else and it's not difficult to tuck yourself away as scum. Hmm, perhaps I should be more clear:
As town, the 8 people around you are all equally as suspicious to start the game. When you accuse someone, there is... A natural amount of hedge, of, "I really don't know". It's often accused that hedging is a scumtell, but in reality they know for certain that their accusations can't miss (town). Because of this, there's a lack of natural incentive to hedge. So I like to look for people who's "I'm really not sure" comes across as disingenuous. It's a lot harder to force feelings that should just be natural.
That said (and moving back into the first paragraph), unless they are under pressure, scum won't lie very often. If/When they start to attack someone it's usually because that person has done something to appear scummy. I use Hedging as an example above for this reason. "You suspected X, and now you don't?" - "You voted for Y when it was "safe", then unvoted when he got extra votes? lolscumbuddies" - etc. They're arguments that are easy to make, but they aren't really scummy behaviors. As town, don't let yourself get drawn into making bad, or irrelevant, arguments - and be on the watch for Mafia doing so.
They may have the advantage in that they can't be wrong, but we can use that to our own benefit. Mafiahunt, don't scumhunt. I also like to look for where actions don't meet words. Someone who indicates a desire to act one way, then doing another.
...And, this post really lost it's flow. That's what happens when you write every other sentence after some sort of distraction. Oh well. I think the gist of what I'm saying comes through. Mafia have a huge advantage in how "not to apper scummy" over the town, so it's important to find Mafia trends and behavior, not scummy trends and behavior.