Im assuming you want to determine dome or dish volume?
Depending on dome height, put the piston down in the hole a fixed amount, set with a dial indicator. (1/4", 1/2", whatever gets the dome below the deck. )
Seal the rings with some grease (that volume can be calculated, if neccessary ). Calculate the volume of the cylinder as if it were a flattop with no reliefs, then cc your cylinder as if it were a cyl head, and compare volumes.
And im betting there is a video on you tube, somewhere.
