2 quick places to look. Intake mounting bolts to the heads. Measure side to side. 9" is a 302 based engine, 11" is a 351 based engine.
Even easier is to look at the distributor to intake. The top of the block right above the distributor to the intake manifold. That edge to the distributor mounting surface. If it is nearly flush, maybe ¼", that is a 302. But if that distance is over an inch, that is a 351 engine.
You cannot go by firing order. The 302 and 351 have different firing orders but a cam swap will change that. the firing order is dictated by the cam profile and the cams interchange. Sometimes a desired grind is only offered in one firing order but not the other. Even Ford played this game when the 302 went to the HO version, the firing order changed to the 351 version. 302 cams were historicly cheaper and more options existed, so a lot of 351s got 302 cams.