I am sure you know to look for the B20 mark and injection ports. To tell the B20E from the later F look at the side of the head in front of the number 1 exhaust port. On the E you will find an auxiliary air valve, in a larger version of the hole used earlier for the oil cooler water. On the F head (74-75) this area has no hole, and is a cast surface. When you have seen enough heads, it is possible to judge the head thickness by looking at the edge of the head beneath the spark plug side, where the head gasket is visible. This can give a rough idea of the thickness without measuring. The early E head is very thin in this area. With the water pump removed, and the thermostat housing off, a good pair of dial calipers could be used to accurately measure the head thickness, and with that number, it is easy to consult the factory dimensions to determine the compression ratio. The final ratio achieved is also due to the head gasket thickness, as there were many variations. Here on the coast of Canada, I use the Volvo Penta marine gaskets as they are really thin.