It is my belief that all the 1:1 trannys drove 14" props, and all the 1.52:1 trannys drove 16" props. So the prop size should tell you.
John has a list of the factory prop sizes for each engine at
http://www.marinette.com/propsize.htm
The tranny also has a data plate that should give ratio and rotation. At least mine do.
The only direct way I know is to put a piece of bright tape on the damper, front of the engine, and another piece on the transmission output. While in gear, jump the starter directly so the ignition and neutral lockout are defeated, and see if the taped spots are going around together.
1975 32' Flybridge Sedan, twin Perkins 6-354 diesels, 1:1.53 velvetdrives, 16 X 19 props. Merritt Island, Florida