Begin by removing the spark plugs, then with a breaker bar and socket, turn the crank over by hand. It should turn freely with no binding, If you encounter a problem, stop and remove the rocker shafts one at a time and try again, if the problem still persists, its time to remove the heads. Which if it were me, I would do as a matter of course, in order to visually check the pistons and valves for damage. Get a good book and take your time, troubleshooting an engine can be a fun winter project.