Actually you can wear out dies. It's not the brass itself, but dirt and little bits of hardened carbon embedded in the brass that does it. It's why you get scuff marks on resized cases: little scratches in the die. Dillon sells solid carbide rifle dies in a couple of calibers for this reason. I have them (.308 and .223) and cases come out looking a lot smoother. They don't wear out in a non-commercial environment.
Welcome to the forum.
The answer to your question is to measure what's coming out of the dies. Use the SAAMI drawings
and a caliper or micrometer to be sure the case diameters don't exceed the cartridge drawing numbers.
Measure the outside of the case necks before seating a bullet and again after seating a bullet. The should grow one or two two thousandths with the bullet in them. If they are growing much more than that, the neck is pretty tight, indicating a worn expander. If the grow very little, then the neck portion of the die is worn for the kind of brass you are using. But the bottom line is does the neck grip a bullet properly. If so, you're good.
If it turns out your necks are OK and the expander is just touching the insides of the necks lightly, that's actually a good thing. Expanders tend to pull necks off axis which can reduce the ammunition accuracy (see this video
). Basically, the lighter the touch the better. If you want really straight necks you need either to use the Lee Collet Die for neck sizing, as shown in the video, followed by a Redding Body Die to size the body, or you need to get a bushing type sizing die, like the Redding Full Length S dies, remove the expander (or substitute and undersized one), then select a bushing that gives you the right final neck size for your particular lot of brass without using an expander (and undersized expander will still remove minor case mouth dents on the way in ).
Finally, since full length sizing is when the necks grow, you normally want to do that before trimming. Most chambers have some spare space, so you can often get away with doing it backward. But since cases don't all grow identical amounts during sizing, the practice can make crimps a lot less uniform, if you use them.