The only way to be sure is to have a factory historical letter stating it shipped to the US military.
Firearm History Request - Smith & Wesson
There have been both Army and USMC model 10s verified, but as David stated the USAF generally used model 15s. Usually this will have a property stamp of some kind, but these are easy to duplicate.
It's a gamble, spend the money for the gun and the letter and take your chances...please let us know what happens.