This thread is one about a guy making a 'cheat sheet' to give to his friend so his friend can help him look for 'good finds' in thrift stores. There is a lot of quibbling about tubing and stuff in there, but if you pay attention to the things that people are suggesting that a non-bike person look for in order to identify good bikes, you'll find some good hints.

http://www.bikeforums.net/showthread.php?t=112907

For the purposes of fixed gear, make sure you have good horizontal dropouts (forged are best), and preferably a crank with a spider, instead of a 'spiderless' crank where the chainring is permanently swaged to the base of the crank. That way you can change chainring sizes in order to find the best gearing.

27" wheels aren't a big problem. Its true that there are fewer rim choices and tire choices, but there is enough to be ok. And if you ever want to put on 700C wheels, they are only a few millimeters smaller, and will usually work fine. Brake reach on the front brake is the only real concern, and usually the old back brake has a longer reach, and can be moved to the front.

Cool vintage paint jobs, chrome lugs, and leather saddles are all x-tra hip finds.

Enjoy!

peace,
sam