I don't think that weight is a good indicator of quality except when the PSU is very, very light, and even then you have to look at the case material, the cables, and the number of fans because they probably affect the weight more than any internal details do. Even the weight of the heatsinks may not mean much because Powmax PSUs have some of the heaviest ones but aren't considered to be very good.
The best indicators of quality:
1. Fortron or HEC is the manufacturer, or
2. There are only three screw holes on the top of the case, provided the PSU wasn't made by Key Mouse (MaxPower, EverPower).