Ok so with my RCBS Die How do I know how much crimp?
For bullets with a cannelure you should be able to tell by looking at / touching a non-crimped cartridge and a crimped one. The brass should be ever so slightly 'folded in' into the cannelure area. How much depends on the level of crimp. Less is typically more, typically no need to go crazy on the crimp.
You do not have to trim your brass to the same length but if you are seating to the cannelure you will get more consistent seating depths if you do so.
Lee makes a 'factory crimp die' for 223 which is just used for crimping. You would seat the bullet without crimp with the seating die, then crimp with the factory crimp die. It has worked well for me.