How do I actually know when I am pushing the shoulder back?
To keep this higher level - assume what I'm talking about is for factory chambers, starting with once-fired matched brass, and using factory FL dies.
The most common answer you're going to see if you search is to measure using headspace comparators. Measure case, size case, look at the change in measurement. An important point for this process is to only use a case ONCE for measuring. Don't keep resizing the same case multiple times to set a die, use a different (fired-unsized) case for each adjustment of the die.
It should be pretty obvious that you're only sizing the neck at initial set up of the die high off the shell plate - there's usually a visible ring of sizing where the die stops. So leave set the die high to only size the neck for as many firings as it takes until you can't rechamber a fired/un-sized case without pushing down on the bolt. Then resize cases progressively shorter until a case fits, only sizing a case once for a test fit then set aside until the end. Resize all the rest, shoot them all once, verify the sizing spec continues to work. If you anneal it should continue to work. If you don't you'll likely need to continually adjust the die down slightly for several firings. If adjusting the die down to the bottom doesn't solve the fit, there's something else other than headspace going on.
You'll know you're pushing the shoulder back because cases will fit in the chamber differently than before you sized them.
I understand that you need to make sure the neck is correct but when do you know if or when you need to push the shoulder back?
Bolt drop test (what Kirby and I generally described above). I strip the bolt down so it moves freely and doesn't engage the trigger/springs. You'll feel a big difference once you fire a case enough times to NEED sizing versus one dimension being off. It can take anywhere from 3-8 firings depending on the chamber to really fill out a case to where it NEEDS sizing to fit.
There are considerations for function though. Minimally sized case using this process can cause you problems if there's any chance of the case or chamber getting wet. To address functional concerns - you can still set the die using the above process, but then use a comparator to go an additional 0.001-0.003" of shoulder movement to put more space between the brass and chamber.
TL;DR - stop here if you're going to complain later. You were warned.
Trim length comes into play here also, depending on if you're using a cutter that indexes off of the case base or shoulder. Both work, but you can cause inconsistent trim lengths if you trim indexed off the shoulder before shoulders are fully grown/stabilized. 30-06 is what gives me the most problems with this - needs trimming much sooner than anything else I shoot. Some cases I never actually HAVE to trim over the full life of the cases, but compulsive unnecessary trimming is a subject for another thread. So save yourself a headache and if the cases don't jam into the lands when you re-chamber them, don't try to work on trim length and headspace at the same time. You'll end up running around in a circle.
All of what I said is based on what I see as one key fact - a single firing will not grow a case enough to NEED a complete resizing. If it does then IMO you've trashed the brass on the first firing with too heavy a load, your chamber is cut wrong versus the brass you're using, or you've spent so much time on a reamer/die/cartridge that it's perfectly dialed in (looking at
@Fiftydriver 
). It matters WHERE on the case the sizing is occurring, but that's more in-depth than your "am I bumping" question. To Kirby's point on feeling resistance after a single firing, there are more dimensions than just the shoulder at play. Case head just above the web is a common location that can be sized by certain dies when shoulders are still short of the max - this can cause headspace to GROW on your first sizing, or at very least become inconsistent when still short of max headspace. That leads you into reamer vs die dimensions, neck vs body sizing, and so on until you're the one writing a dissertation that ends in needing a custom die for a $799 Savage Axis II scoped combo rifle because you just know it has a 100-10x in it.
