There sent to a test range in wiltshire. The ballistic vests are tested with various calibres of rounds and at different velocities ie, a 7.62 at 650 ft/sec to find the NIJ levels of each product. As for stab vests. A projectile (edged blade, hypodermic needle, steel spike ect.) is attached to a numatic ram and fired at the vest. The helmets are tested in a similar fashion to the ballistic vests.
I belive there are videos on you tube of us testing our EOD mine clearance suits. We blow the f**ker up with something like 4lbs of C4 and there is hardly a mark on it! If you type in "United Shield International" you should find the vids if your interested.