This is a highly scientific proceedure that I use. If I feel the primer has slipped into the pocket too easily, I hold the case, primer down, at about a 45deg angle and tap it sharply on the desk top (wooden) a couple of times. If it doesn't back out, then I'm satisfied its safe. Also, after you have fired the rounds, look for black tracings around the primer. The case and primer are seated against the face of the bolt but if the primer is loose, a bit of carbon will seep out between the primer and the case head.