OK, a marine engine probably has the cam gear driven. If you have mechanical fuel pump mount, pull that open, look in there and see if it's chain or gear.
Probably the best way is to run no1 cylinder manually through a firing stroke. Pull the valve cover, get a socket on the front bolt, and bring the timing marks up with both valves closed. Now just slowly rotate the engine, and visually watch the no1 valves. A normal engine rotation, starting with TDC, which is "combustion, will go down 1/2 crank turn (180*) and at some point the EXHAUST valve will open, then another 1/2 turn will bring piston back up WITH THE MARKS AGAIN at TDC except now both valves (near TDC) will be slightly OPEN (split overlap) At this point the exhaust will be CLOSING and the intake valve will be just starting and OPENING. One more half turn (180) from the TDC marks the exhaust will be finished closed, the intake will open and close, and finally, another 1/2 turn coming back up to TDC both valves will again be closed and back at your starting point.
A REVERSE ROTATION ENGINE WILL NOT follow this sequence.
You may not be able to tell by the distributor, because if the cam is gear driven on a reversed engine, the oil pump and distributor will be driven the same direction as a vehicular engine.
THE DISTRIBUTOR YOU PICTURED is for CW rotation of the distributor. Here's how you tell on ANY distributor:
"Spring" the rotor and see which way it moves. If you have to move it CW AGAINST THE SPRING, and it "springs back" CCW, then it is for CW CLOCKWISE rotation.
Look at your vacuum advance. When that advance lever moves, it pulls the pickup coil CCW WHICH IS THE SAME RELATIVE MOTION as moving the rotor CW
THAT DISTRIBUTOR is for CLOCKWISE rotation.
THAT DOES NOT MEAN that it came out of a "normal" rotation engine.
