I'm mostly speaking in a very tongue-cheek-manner here, but in all honesty, I do not believe in what passes for typical scumtells. In fact, ironically, I tend to find scum using traditional scumtells more than anyone. (Which I guess makes that a scumtell, kind of?)
I'm at my best when I can work out how a person thinks & reacts to things, and then look for patterns from there. I'm not going to say scumtells don't ever work, but like any tool, they're very situational, and most people don't seem to get that.
Basically, understand the person, look for patterns, and the rest follows. I don't even need to talk that much to do it, which is nice, since I'm not a great conversationalist in Mafia.