I once read (maybe Randy's site) that if a bike has a brand name that you never heard of, but it's a foreign name that ends in a vowel, then give it a second look.

Signs of vintage entry level bikes: turkey levers (extensions off the normal brake levers), stem shifters, big metal or plastic dork disks (between freewheel and spokes on rear wheel), protective rim outside of big chain ring. Lots of exceptions, and those things can be removed fairly easily, but usually original owners, and lazy bike flippers, leave those things alone.

If it has a sticker on the seat tube (usual location) that states the tubing type (531, Columbus, Champion, etc), then that's a good sign. The list of quality tubesets is much shorter than the list of quality bike models.

The components are a whole additional huge topic. Generally Campagnolo was on the better bikes, but again that stuff can be stripped off, so a great frame may end up with lesser components.

It's a deep subject. Probably the quickest way to learn is post on the appraisals C&V forum whatever cool bikes you see on Craigslist, and read all the threads you see like that. But do a quick search here first, you'll start figuring out a lot of it on your own.