Buying used heads?

I have a chance to get some 516 heads (yes, I know-hardly the best choice) but I can get then very cheaply. The seller states that they have been professionally ported. But after talking with him on the phone, he admitted that he had done the porting himself, following instructions from an un-named 'pro'.

Well, a beginner at porting can sometimes ruin a set of heads rather then helping them. I'll be looking at them tomorrow. What should I look for to tell if they are usable? Too much off the short side? Too much out of the bowls? Runners too big or possibly punched through? Any advice?