You can tell by CCING the heads, and comparing what you get using some colored liquid, vasoline, and a cd case over with a CC syringe, less than ten bucks, get some numbers that are pretty close. .
The best thing i ever did to my 318 was mill .045 off my 302 heads. reducing them to around 58CC. Also took .045 off my 360 heads, and .100 off my 225.