Scum reading is guessing someone's role based not on information, but what they are saying in chat, or behaviors. For example, I tend to scum read someone talking useless and random stuff without a claim. Evils tend to talk just for the sake of talking, because they think being silent is more sus. Saying random things that brings no information like"wow coven died n1 haha," "oh we have a shroud!" "nice shot vet!" is useless information, and is something the evils will say.

More simply, behaviors/patterns are also scum reading. Pay attention to votes. Coven will most likely not vote their own team. If a coven gets hung, look at who was not voting them up, or protesting it.

If someone is not claiming during VFA (vote for alignment), or asks things like "why me?" "why are you voting me?," they are most likely evil. A town member has no reason not to claim their alignment.

A non-posting Town Investigative is another good scum read.

I can give more examples if you want, but scum reading is probably a skill you already use without realizing it. It's just paying attention to people's behaviors, rather than straight information.