Lots of ways to do it with tools, but I just press a small dent in the casemouth, press fit a bullet, and chamber it. The lands will usually push the bullet back. Try it a few times and measure the OAL, as the lands may sometimes grab the bullet when you retract the round and pull the bullet back out a bit. Then I load a dummy round a bit longer than that on my press and try to chamber that. If it won't chamber, I bump the bullet seating down a bit at a time until it will chamber. If it sticks, I tap it out with a cleaning rod. Once it will close with a bit of resistance, I mark the ogive with a sharpie and chamber agan to see the marks that the lands leave. You can wipe off the sharpie with alcohol and repeat while shortening the depth until you get very short marks, indicating that there is just a bit of land contact. That's your new dummy round and the OAL to work from....
