We used to be able to check iphone 3G part numbers (MB048LL/A, etc...) and whether the part numbers are locked to carriers from this apple site.
Locating iPhone wireless carriers

Seems like the page has changed and we can't check part numbers anymore.

I'm about to get a factory unlocked 3GS overseas. Is there any other way to check whether the 3GS part number I'm purchasing is/isn't locked to carrier?