There's cratering and then there's
excessive cratering.
Cratering happens because high pressure causes the primer cup to flow around the firing pin and into the hole between it and the bolt face. This can happen for any or all of three reasons. First is an excessively large firing pin hole hole, which leaves a gap around the edge of the firing pin itself. Second is a weak firing pin spring. Third is excessive chamber pressure.
The key to reading excessive pressure signs is increasing the charge weight gradually, examining each case after firing (I use a 6x hand lens) and looking for the onset of each "sign." You really have to get to know the individual rifle, as well as your bullet components. Like Joe, I have a .204 Savage that craters with relatively low pressure rounds, even with factory Hornady loads. And some primers have a reputation for being either "hard" or "soft." Soft primers are the ones that will show cratering at lower pressures.
Besides cratering, you need to also look at primer flattening. Most primers have less rounded shoulders now than they used to, but even the new ones will get flatter, with more squared shoulders as the pressure rises.
Pressure gets higher as you move left to right:
Notice how the appearance of the ring between the primer and the rim gets narrower and shallower as you move right, indicating the top of the primer is getting wider (and flatter). This is consistent with increasing chamber pressure. Notice, too, there is no cratering in the leftmost round. Second leftmost, slight cratering. At furthest right, the cratering mysteriously has vanished because it smacked the bolt face
so hard, it was smushed back into the primer. I suspect this primer also is "top hatted," meaning it now is wider at the top than at the bottom, like a top hat.
This is a slightly top-hatted primer:
IMHO, top hatting always is a symptom of excessive pressure.*
Cratering itself might not be conclusive but
excessive cratering is a prelude to primer piercing. And it is possible to get pierced primers before you get pronounced flattening (usually because there's something amiss with your firing pin).
It's also possible to get blown primers, which literally will fall out of the brass when extracted. Or in less severe cases, the primer pockets could just get looser the first few times they're shot, which you'll notice because it takes less pressure to seat the new primer.
*But all these primer problems also can be caused (or contributed to) by excessive headspace.
If you've approached it cautiously and loaded incrementally hotter loads, you can tell a lot from the change in the force required to lift the bolt handle with subsequent loads. Under such high pressure, the rim of the case also will flow into any available depression, like the cut-out for the ejector. Once the rim has begun flowing into the ejector, the case will begin to bind against the bolt face, increasing the effort needed to turn and open the bolt. A little bit of extra bolt lift effort isn't necessarily a bad thing but that's a decision you and your rifle need to make together. If you have to use a rubber mallet to open the bolt, ...that's probably a little excessive.
This "flowing" also will leave a distinctive mark on the casing, known as ejector smear (or swipe):
This one was fired in a semiauto. The smear in bolt actions usually is shaped like a hemisphere or a tombstone. Soft-ish brass can show ejector smear before you've reached the point of stiff bolt lift.
The bottom line is nothing can beat the experience you'll gain by loading incrementally hotter, approaching a max load slowly, and learning the order and at what point the pressure signs appear in your particular rifle. And just because you've reached the onset of a single sign of excessive pressure, that doesn't necessarily mean your load is unsafe.
The picture of the smear mark is a good example. That primer is still severely rounded, which would lead me to speculate that this brass is soft-ish, because the primer shows no hint of cratering and very little if any flattening.
Some people use case head (or webbing) expansion as an indicator of excessive pressure. But it's unsettled whether this really is a reliable indicator, plus it requires being able to accurately measure to the ten-thousandth of an inch. But it is a quick and dirty test of webbing expansion to try to fit the just-fired case into a case holder. If it won't fit, that's a pretty good indicator the case webbing has expanded too much.
A chronograph also is a great tool for double-checking your pressure observations.
