Tommy Silva just raps his knuckle on the wall and voila, stud. Of course this probably works on drywall alone. Maybe tapping a hammer on the plywood will ring differently when you get over the stud. Hunting for nails or screws through plywood and drywall is a fools errand without the high price product, I would imagine, as stated already.
NOTE: since the plywood is over the drywall as you state in initial post, aren't the fasteners in the same line on the stud, and can't you see the fasteners with your eye???!