I thought I flattened the primers on some of my first reloads ....even though I was loading light.
Learning to read the overpressure signs is one of the more challenging knowledge/skills that makes reloading a rewording and challenging hobby for me.
I'm not even close to an expert, but here's what has helped me.
Get a good reloading book like the ABCs and study the photo's carefully. They go to some length to illustrate normal, hot and dangerous deformations.
Look very carefully at the fired primers on factory loads. They all flatten some, but when they retain their curveture at the perimeter, that's normal. When the whole circle is smashed flat as a pancake right out to the extreme edges....your hot.
Save your brass and compare fired cases from different factory loads (CCI, WIN, Fed, Rem....).
Look to see if the primer has "just a dimple" (it wouldn't have fired with out the firing pin striking the primer), or is there a deeper hole with the primer formed distinctly into the shape of the firing pin.
If you ever get a FTF (light hammer strike or hard primer that fails to detonate) study the cartridge carefully before you give it a second tap. This dimple is abnormally light, as there was no pressure from the detonating powder pushing the cartridge back onto the firing pin.
It's this kind of "wisdom from personal experience" that you can't completely get from an internet posting or book that makes reloading fun for me.
But, like all worthwhile things, it takes an investment of your time and doesn't happen over night.
