GP9 and GP18s are very closely related and the easiest way to tell is to look at the roster of the railroad you are interested in. There are some features that lean toward one or the other but the parts are interchangeable and over time it can be very hard to tell the difference. I agree with Terry though, this one with the diamond shaped wire pattern in the radiator intakes looks like a late GP9. As an aside, the original Norfolk Southern had low nose GP18s. When it was absorbed by the Southern Railway, they were rebuilt with high short hoods. Another aside, I think it was Train Station Products made radiator intakes to modify a Proto 2000 GP18 to resemble a late GP9.