Lisa has the right idea. Avoid American made bikes from the 70s and 80, as most were made with cheap welded steel tubing and one-piece cranks. An exception would be a Schwinn Paramount which was made by Waterford. If the frame is lugged, that's a sign of quality. Cotterless cranks were also a sign of quality in the 70s, but became ubiquitous in the 80s. The overall weight of the bike is important, but expect a 3-speed with fenders to be heavier than a racing road bike. I can give you more hints if you tell us what you're looking for: a vintage bike to restore, something to learn bike mechanics on, a beater bike, or a sleek road bike. If you want something to fix up, then the availability of parts will be important. French bikes use different threading than is standard today. A few things to look at:

The crankset:
- avoid one-piece cranks (see http://www.sheldonbrown.com/opc.html). They are not interchangeable with modern cranks because of the bottom bracket shell.
- Three-piece cottered cranks (http://www.sheldonbrown.com/cotters.html) were once standard, but cottered cranksets are always steel (ie heavy) and can be difficult to overhaul.
- cotterless cranks (the 2 left hand pictures here http://sheldonbrown.com/gloss_bo-z.html#bottom) are alloy and most desirable, and should generally be interchangeable with modern parts.

The wheels:
- steel rims on an old bike will probably be rusty, and steel rims are heavy
- alloy rims are better
- rim/tire size - some are harder to get than others
- hubs should be overhauled with new grease
- no broken spokes, true rim - really old bikes may have frozen spokes (ie nipples won't turn) and be impossible to true.

Also check that the frame is not bent anywhere. Fork get bent easily and often people ride them that way for years.