By far the easiest way to tell is to take off the inlet manifold. Bigport ports are not evenly spaced (1 2 . . 3 4) while smallport ports are (1 2 3 4).
Or if you have an original wiring loom read the dates off any of the wires (they can be a bitch to find, but every wire should have a year printed on it somewhere).
Hen
