I'd say it's on the paper, as several others have noted.
Originally Posted by jaydebruyne
If you wanted to do a conclusive test, and don't mind wasting a sheet of paper, you could dial the enlarging lens out-of-focus, then make a print. If you still get the white specks, you know that they can't possibly be dust on the negative ('cuz it would be too far out-of-focus).
I don't know your printing conditions, but if there is any open woodwork or the like above your enlarger, you might try taping a sheet of plastic dropcloth to the ceiling over the enlarger. This would stop debris from falling straight down.