According to the new Kindle User's Guide, 4th Ed., one way to tell if you have the US or Global Wireless versions that will work if you don't have a skin making it impossible to read the model number on the back (just below the serial number). However, this may require a magnifying glass if small print is a problem.

So far, I think the serial number is easiest.

Model Numbers: --
Kindle - D00511
Kindle with international wireless - D00701
There are a few different ways that were mentioned in the thread
Kindle 2.3 update where I posted the info above. For example:
Info on back: serial, model, FCC-ID
611 from Settings and check various information
Display when in slow WN area (US displays 1X while the International displays EDGE or GPRS)