One of the quickest ways to determine which size case is to look at the shape of the pan gasket. Another quick way is to look at the casting. If the casting has holes, as shown below, it’s a large case. No holes, it’s a small case.
After identifying which size you have, you will need to determine which model. An easy method is to notice if the internal solenoid wires extend outside of the case. Another way is to see how many holes appear in the circled areas shown.