Go up and tap it, if it is fibre glass it is a fake, if it is metal, it is almost 100% likely to be real. However, there are some REALLY good replicas out there that are identical to the real thing barring the fibre-glass body, so is very difficult to tell if it isn't on an age related plate (that shows it being newer than late 60s) .

Also the price tag will give it away (if it's for sale), as the genuine ones go for £750k .