Correct way to measure piston depth?

thanks again Bobby. You are just a wealth of information and advise! I will proceed with caution for the time being. These 302's that I have are a newly recondtioned set (by autozone..yea I know) that I got for $100 off craigslist. They look perfect although I am going to try my hand at gasket matching and pocket porting (I will practice on my stock heads first). How do I determine the cc of the heads? Pour water in the chamber until level then pour it out and measure? Inexperience question I bet. Thanks again!!