Remove the rocker boxes, see if all the valves close, if they do, try to rotate the engine.
If it has adjustable pushrods, just pull the top cover from the rockerboxes, remove or loosen the pushrods (depending on what type they are) and see if the valves are all seated.
If it has adjustable pushrods, just pull the top cover from the rockerboxes, remove or loosen the pushrods (depending on what type they are) and see if the valves are all seated.