Look at the "born on date" that is located on the slide behind the ejection port and just above the extractor. There should be an oval w/ an inspector's stamp and a two digit number, say for instance, on my P01 it shows(06) meaning 2006. Also, if you bought it new or if used and in the box, check under the foam and see if the factory paperwork is in it. It's fairly comprehensive in all that was done prior to leaving the factory. For instance, it reads that my P01 passed inspection on June 06, 2006 @1413 hrs. I hope this helps a bit.