Be sure the spark plugs are out when you try to spin it. I don't know if it makes a difference which way you try to turn it, but I'd go the same direction that the motor turns = counterclockwise as you look at the impeller from the rear. Look for signs of water on the spark plugs, or rust...........bad sign.
You may have to try to turn the drive shaft with a wrench. It is difficult to do by hand.
If you can't turn it, unfortunately the next step would be pulling the engine (hoist) and tearing into it.