I've puked a handful of times from exercise, never passed out. Had a team mate pass out about 5 feet away from me, from sprints/plyos.
I get to the point where I feel like I could almost black out quite often, yesterday running I was very close. One day it will probably happen. I don't get like that from heavy lifting much anymore. See lots of stars from heavy lifting though.
For me, I know I'm working hard when my face starts to go numb.