I like to sharpen the end of my paper clip hook, as it makes it easier to feel the sharpened point hook drop into the thinned or grooved part of the case.
Many times you can feel the groove before you see evidence on the outside.
Brian gave some very good examples in his photograph, very good!
Now, one thing that can bring on head separations, other the very high pressures of course, is impropper brass sizing.
Always and forever, size bottle neck brass the minimum amount that will allow brass fired IN YOUR CHAMBER, to again chamber IN YOUR CHAMBER after sizing. If your sizing more that that, you are shortening your brass life big time.
I have given examples and reasons for this - manufacturing tolerences - so many times, I finally just made up a document on the subject.
Not talking neck sizing with a "neck sizer," but proper adjustment of "full length" dies (your die) to properly size for YOUR chamber.
Anyone interested in seeing it, just send me a PM (personal e-mail) with your "E" address and I'll send the info your direction.
Crusty Deary Ol'Coot