What is the proper way to remove plywood subfloor?
If you find the lines of nails/screws hard to spot, try a magnet. They should be pretty easy to figure out, as they will spaced much more closely along the joists, and should make lines that show you where the joists are.

Don't set your saw too deep - it's problematic to cut where you can't see.

One method is to use a small hole saw and drill around the nails, just the thickness of the floor. Then (after getting the sheet of plywood up) you end up with a bunch of small plywood plugs you can break off the screws/nails fairly easily, and enough nail/screw to get a grip on for removal.