They claim (they being the people I've found on Google) that an RFID passport has a special marking on the front. Looks something like what's
here
From what I've read, RFID passports are only being issued in FL and CA currently (but that's hearsay from former clients). If you get a passport now, without that symbol on it, you don't have an RFID tagged one - mine doesn't have the symbol, and it is identical in every way to ones that were issued 6 years ago.