He didn't ask how to do it acurrately, he asked how to find out ;-)
If you don't wanna use water, another things you could do is,,,
get an old spark plug, bore it out, just like coring an apple ;-),,, fit a balloon to it and, with the engine in the same position BDC (bottom dead centre),,, fit the spark plug and do 1 revolution of the engine,,, the balloon will take the air from the barrel,,, size your balloon against a factory set balloon, and there ye go,,, no need for expensive engine rebuilds :-)