How to determine size of engine?

I recently bought a 1969 Ford F250 camper special. The guy I bought it from says it has a 390 in it. The VIN says it has a 360 in it. It is a ford motor from 1969 though. So it may have been swapped, I suppose. How can I check the size of the motor myself to verify if it is a 5.9 or a 6.4? I'd hate to go gettin parts for the wrong motor.... Thanks, Jake