A traditional way is by focussing on something that makes it easy to measure scale (a ruler), measure the magnification (= size on the screen / size in real life) on the focussing screen, and measure the bellows draw.
Then move the thing away, refocus, and measure both magnification and bellows draw again.
After that, a bit of math will produce the focal length.

Focal length = Difference in bellows draw / (Magnification1 - Magnification2)