LEO,
I do EXACTLY what you proposed. Resize a piece of brass, JUUUUUST seat the bullet, then toss it in the chamber and close the bolt. when you extract it, get your calipers out and measure.
Look at the bullet because sometimes with light varmint bullets you won't actually touch the rifling.

If you DON'T have calipers, go to NAPA. You can buy a digital caliper set for $15.