I look for excess or deficiency of material in the welds and/or brazing. I also look for proper tubes that haven't been crushed or dented. I also look for symmetry in the frame. Asymmetry of more than a small amount is often a telltale sign a car has backed over the bike and someone has tried to straighten it out poorly.

I also look for straight forks and clean drops that don't have serious dents or deformities.

I'm not really looking for perfect, but what I'm doing is eyeballing for damage or cheap workmanship.