Rake your gravel using a landscape rake, evening out any dips you encounter.
Lay a piece of wood across your driveway so it stretches across the entire width.
Place a spirit level on top of the piece of wood. Check the level. If the bubble is in the center, your driveway is level. If the bubble is not in the center, something is off.
Add more gravel as necessary until the bubble moves to the center of the spirit level.
Move the wood down the driveway 1 inch at a time. Watch the bubble to ensure it stays level. When it moves out of the center, you've encountered another uneven spot. Fix it as you did the initial one and keep going.