I figure it by magnification as mentioned above (the ruler in the scene measured on the groundglass), but I don't always put the ruler in the scene if I think I can estimate it based on knowing about how big the subject is compared to the size of the format (i.e., a portrait where I can see both shoulders maybe with a little extra space is about 1:3 on 8x10"), but with higher magnification I'll measure it. I have a table taped to the backs of all my cameras, my light meter, and in my notebook to convert magnification factor to exposure factor. I've posted the table around here somewhere as a doc file.
For small formats, you don't actually have to measure on the groundglass. Just make sure the ruler covers the full width of the frame.
if john cook were here he would chime in, between yarns told about the days working in hollywood
he made this post which is part of the large format site
scroll down and you will see it ...
its a little different than what rick said but similar ...
its pretty easy ...