You can tell by looking at the trigger. There is a little pin in the side of the trigger that holds the inner "safety trigger" in place.
On the original trigger this pin is close to the tip of the trigger while the "upgraded" trigger has this pin in the middle of the trigger.
Here's an original trigger:
Here's an upgraded one:
As far as I know they upgraded both .40 and 9mm but I think the problems where with the .40 (if not both).