High-quality electronics increase a radio's value, at least to collectors interested in the technology as opposed to mere appearance. Some high-end brands, such as E.H. Scott or McMurdo Silver, were very expensive when new and still command higher prices than their humbler brethren. Some of these sets are easy to identify. Some sets have two chassis instead of one, and chrome plating is another tipoff. If you see chrome on the chassis, metal components, speaker frames, etc., you are looking at something out of the ordinary.
Nice radios are generally worth more than beat-up ones.
Most importantly, the set should be in complete and original condition. Award bonus points for original packaging, manuals, and accessories. Subtract points for cabinet damage, missing or incorrect knobs, and any amateurish modifications. You can get modern reproductions for many knobs or dial covers, although repro components may reduce the set's value to purists. Cabinets typically show some marks of use, and a few minor scratches and dents are to be expected.