DF,
It's extremely easy, and only slightly messy:
1) Use fired cases, since they better represent actual powder room.
2) A bullet can easily be slipped into the neck of fired brass. Or at least it should. If not, there's insufficient neck clearance for some reason, usually a too-long case.
3) Fill the case with water to the brim, then set it on some flat surface and carefully insert the bullet. The excess water will flow out of the neck around the bullet.
4) I usually use a cannelured bullet, since it's easy to seat them to the same depth when comparing the capacity of two or more cases. But you can also use a Magic Marker to make a witness mark.
5) Remove the bullet. Often a drop or two will stick to it, so I scrape them off on the edge of the case mouth, then wipe down the outside of the case.
This shows the actual amount of space available for powder with that bullet seated. Filling to the brim gives a few grains advantage to long-necked cases, where the neck is filled by the bullet.
