If you encounter a unit you can’t identify, fire a shot above their head so it won’t hit anyone.
If their response consists of rapid, precise, and controlled fire. They’re British.
If their response is a shitstorm of machine gun fire. They’re German.
If they throw down their guns and surrender. They’re Italian.
If nothing happens for 5 minutes and then your position absolutely obliterated by support artillery and air strikes. They’re American.