Is your motor apart? or are you trying to determine this with it together? I’m not sure there is an accurate way to determine that with it together. You would usually compare the advertised or measured cc’s of an untouched head’s chamber with the chamber of your head. My heads are not on a motor. I can measure from the deck surface of the head, but to where? If you are looking for an overall height measurement from the oil pan rail to somewhere on the head. That won’t tell you the head was milled. Could be block deck/head gasket etc. Why are you trying to determine if the heads are milled. To calculate compression? To see if the intake will fit fine? To check piston to valve clearance?
