Just an educated guess, but the first serious revision rolled out with the Xray sights around May of this year. That was to address the battery issues, broken strikers, spring issues, etc. Quiet changes were added (not considered a formal change) by mid to late summer including what some report as chamfered firing pin hole, so anything in the Fall and onward should be as new as it gets.
That being said, mine is first batch after the first revision and I've had no issues at all other than minor things that I would say are more break-in than anything else. However, I'm keeping my eye on that striker.
If you're paranoid, make sure the date is at least July/Aug. If you're not too worried about it, then just make sure they have Xray sights to signify you have the first revision at the very least. I think they used to use Siglights on the very first batch, so if you're being offered Siglights, pass on it.