I guess I should explain how I did it (after reading some article on it many years ago). I always used the same primers (Remington and Remington magnum). I made sure the factory ammo primers (Remington CoreLok) weren't flattening. Then I worked up my load (very carefully) until it "appeared" that the primer corner was starting to flatten. This was never severe as I didn't jump powder charges up much. When I hit that point where it "kind of" looked like they "might" be flattening I backed down to the previous load and that was it. I still always watched it when shooting at the range. It seemed to work good because I could always get pretty close to or even at the max. charges. Always used the same brass too (Remington) and the same bullets (either Nosler partition or Sierra).
From John Wooter's safety tips...
Watch for signs of high pressure while working up a handload. This means extraction difficulty (however slight), flattened primers, cratered primers, ironed-out headstamps, polished headstamps, ejector marks, case-head expansion, and excessive recoil and muzzle blast. And anything else whatever that strikes you as abnormal about the load.