I am going to laydown wood flooring and notice that in one area that the subflooring is curved upward like a hilltop. I don't have anyway to go underneath the joist to look at the problem since it is on the second floor. Is there anyway to level this out before I laydown the wood flooring?
This often occurs at beams that are upset into the floor joists. It is always a challenge to fix, but you're definately right in wanting to take care of it prior to installing wood flooring.
My suggestion would be to use a floor sander or a belt sander with a 40 grit belt. You can use it to reduce the height of the hump by thinning the plywood or OSB at that location. I'd suggest using a long straightedge to aid in the process. Spraying marking paint on the areas that need to be cut down will aid you in determining where you've been an how much you're taking off.
You need to calculate how high the joist or beam is relative to adjacent area, say 4' on either side. Does it occur in middle of room;adjacent to a wall; does it extend thru a doorway into another area. These conditions will dictate your approach to problem. Use an 8' straight edge to calculate relative height equally on both sides then with the center of your straight edge on the crown bottom one side and measure how high the opposite end is above the subfloor. Repeat process in opposite direction.
Level isn't important here; amount of crowning is.