Another method is to read their history of posts- if one exists. Seeing how they relate to others in generic postings is a useful indicator to determine how they will behave in a team.
I've had probably around 100 offers to help with projects through the years. I spend tons of time getting them up to speed only to have almost all of them disappear. I think Squall's 'pro team' opinion is very accurate, but unfortunately, it's very rare to find someone who follows through. That reality has left me with the same 'jaded' outlook as Psyklax; it's easier to just do the work myself.