When I'm concentrating and keeping my mind distracted, I can go on for quite a while without realizing that it's time to eat, I can completely forget about it. But when I become aware of the hunger, I start feeling sick (lightheaded) after a short while if it's been long enough since eating something, so that's a sure indicator. Oh, I can still go on for a long time, if I manage to distract my mind from it again, and get things done (can't guarantee that the quality of the work is all that great, though ).
I can tolerate high levels of pain as well. For me, the trick is not to concentrate on the pain at all, so it doesn't really go away, it sort of subsides but stays at the back of the mind and it doesn't bother me that much.