For me the symptoms are drowsiness, difficulty concentrating, super minor irritation of the eyes, and if it's bad enough, microsecond lapses of consciousness like a jump when two frames are missing from a reel of film: probably microsleep.
In middleschool I once had fever so bad I couldn't stay awake, nodding out at my desk as if high on downers. One of the few times I ever went to the nurse: didn't want the school authorities to suspect me of Seconal abuse.
As I write this at the Salt Lake City airport, the sky outside is white with blowing snow, I'm sleep deprived and stressed from overwork, my eyes are irritated and the microsleep bursts are starting. I'll be visibly sick with flu by the time they get me home.