As mentioned above get a long straight edge to check the surface. You use the edge to look for a gap underneath.

If you have bought a new sheet of ply for the top of the bench ,just start shimming any low spots.

Depending on the thickness of your plywood you will probably just need to make sure the corners are not wrong in relation to the middle. If you have a low corner start using something close to the right thickness that shouldn't compress much later. Typically I have used thin cardboard or even folded paper. for small corrections..

Personally when I resurface a building table I like to use countersunk screws around the edge to make sure it stays where I put it. just double check to see if you have tightened anything to much.. And caused a low spot.