If your hood is a good reproduction, there may not be a sure way to tell it from the real thing. Do you have any reason to believe it is not GM? Can you contact previous owners and see whether any of them replaced the hood?
In their day, these hoods were available over the counter at GM Parts Departments; they were popular. It is possible the hood was purchased over the counter at Chevrolet. That would make it genuine GM, but not original for your car.
GM hoods were press molded gray fiberglass. I do not advocate scraping paint and primer off your hood, but if you find a spot of bare fiberglass, the color could be a clue for you.