I always check the build order of the low-rank and watch him for the first two minutes. That's usually enough time to check if they're some Brigs second account or an actual newb.

Exile 3v3 game earlier today landed myself a cutter recruit. Went Barracks - Reactor - Supply - Supply before the game finished (short game, like 9 minutes) and he only ended up with a small collection of marines. Funny part was we could hear him arguing with his parents over the microphone and he sounded like some 12 year old brit.