There is no way you can pick out a scrub from a vast amount of people these days. Even with pugchecker and the Gearscore addon, gear is just about free to get and scrubs can get lucky and get pugged into more than half a guild run. So when you check what they've done they could have gotten carried through 25man ToC and do shitty heals or dps and die to the most stupid shit. So if their GS is high and they've cleared the place, you better hope they can carry their own weight :P