The only way to confirm a bike fit is to test ride the bike. Otherwise, you're either guessing, or making calculated approximations. Of course, the latter is preferred, but even then, sometimes, mistakes are made.

So therefore, one method of making a calculated approximation is thru the use of a bicycle sizing chart. Here's one for mountain bikes:

Mountain Bike Sizing and Fit - What is the Right Size Mountain Bike for Me?
(Mountain)

Here's one for road bikes:

http://bicycling.about.com/od/howtor...ike_sizing.htm
(Road)

* Just remember:

Some people have longer legs, than torsos. Some people have longer torsos, than legs. I've even seen people with extremely long necks and heads too!

Also, each bicycle company has different standards by which they assign bike sizes. For instance, a size 54cm made by Trek, may very well be a size 56cm made by Surly. Therefore, bikes sizes don't necessarily have to directly translate from bicycle company to bicycle company.

I guess, just like people, bicycle companies are different too!