Had a buddy who popped for speed a few years ago. He claimed it was from his pre-workout (forget which one it was), and the commanding officer backed him on it. The lab was able to show the difference in results between the ingredients in his pre-workout and the actual drug. Turned out he had taken an unknown pill from a chick at a party and it was ritalin or something similar. Sucked, because he was a good dude. Just made a bad choice and got discharged.
Point being, though, the lab could tell the difference. So ever since then I've questioned the old, 'my friend was taking preworkout XXX and got a false positive for amphetamines'. This was either a Navy/DOD run lab, or a private one contracted by the Navy, I don't know how the actual testing is conducted. But any professional lab should be at that same level.
Now you just have to hope that what is on the label is the only thing in the bottle.