Honestly @fly , if it is just small up/down low spots you want to catch so your floor boards don't flex(or scoot if no glue) - just crawl around with a straight edge and look for low spots. Use a 10' drywall knife(trowel) or if the floor has wider waves a wallpaper trowel(about 24") work as a good trowel for the floor leveler. Or just use the board and wipe clean periodically. *Setting a small flashlight on floor on side of SE away from you will make low points apparent under edge. It ain't rocket science.