By looking at it you mean? I'm not the best person to reply this but i think the biggest way of telling is the inlet manifold, the 320 ones are slightly smaller and less curvy than the 325 ones.
320 has white injectors and a 325 has blue injectors.
Thats the quickest way to tell from looking at it, but thats no guarentee that the bottom end is the correct one.
Someone may have just swapped the top