How can you tell the new trigger from old?
You have to manually check the reset. Dry fire the gun, keep the trigger held to the rear, cycle the slide enough the re-cock the striker, then let up on the trigger. On guns like the Glock and the XD you feel and hear a distinct "click" when the trigger resets. On the M&P you don't. So far I haven't encountered any M&Ps with the new trigger that supposedly has a positive reset.