Lugs - These days just about any lugs are a sign of a decent frame, but in the past there were plenty of low quality lugged frames on the market. Bulky, straight cut lugs are many times found on inexpensive frames not worth a high asking price—pointed lugs are more commonly (but not exclusively) found on quality frames worth bringing back to life.
Headset - Just about any vintage bike will have a 1” threaded headset. Avoid bikes that have a knurled adjustable nut, as they weren’t quality bike when new and are certainly not worth more than a few of today’s dollars.
Rims - Steel rims are bad news. Not only do they not stay in true as well as aluminum rims, they have very low braking power especially when wet. Bring a magnet with you when in doubt—if it sticks leave the bike, or at least budget for new wheels.