Everything you've ever wanted to know about headsets:

http://www.parktool.com/repair/readhowto.asp?id=68

At the bottom is a list of dimensions to measure in determining headset sizing. Having the right tools to measure headset dimensions is critical. For just $2, you can pick up a set of calipers that'll let you determine the difference between 30.2 vs. 30mm and 26.4mm vs. 27.0mm. Harbour Freight - Centech 6" Utility Caliper.