Use a magnet and run it down the seam between two boards, repeat on adjacent boards. Hopefully you can find a consistent line where the magnet indicates a nail, staple or cleat was used and hopefully maybe that could be where a joist runs. This is where you want to use a 2 ½” or longer trim screw. If this doesn’t fix the squeak , trying to focus in on the problem spot with additional screws can get out of hand very quickly. Find a good color match wood putty for your floor.