I use the rear dropouts as an indicator of quality. Invstment cast dropouts are only used on mid-high end frames. Stamped steel is used on cheaper frames. Fancier dropouts=higher-end frame. It is very rare for a high end frame to use plain dropouts.
Straight-cut lugs are low end. If the top of the seapost has a stress-distributing curved end, that is a good quality mark.
Rans Enduro Sport, Hase Kettweisel Tandem, Merin Bear Valley beater bike
I think that it comes down to materials and craftsmanship.
Better quality frames will usually have a decal that describes the tubeing that was used. You'll have to look it up to see where the tubeing fits on the food chain.
While fancy lug cut outs are an indication of craftsmanship, I don't think that's a perfect indicator of frame quality. Many of the better steel tube sets today are designed to be optimized with tig welding. The most exotic frame that I've ever owned was fillet brazed Nivecrom with very thin wall thickness.
Quality of the paint job or tubing decals are an initial tip-off, but they may become marred over time.
I also look at the finishing work done before the painting, filing and cleaning around the brazed junctions. Obviously more fancy, cut-out lugs are an indicator. As are the finished ends of the seat stays and their junction at the seatpost.
Just tapping the tubing with your fingernail and listening to the "ring" can tell you something, too.
As for forks, look at the dropouts. How well finished is the dropout to fork end interface? Is there a sloping crown or flat crown. Details on the crown and cutouts? If it's a unicrown, are there any indicators of tubing. Sometimes the steerer tube has stamped markings of the tubing type, but you have to disassemble to see those.