A quick way is to see if any rocker arms are super loose under the valve cover, if they are, chances are there is a bent valve holding the spring down away from the arm and cam.

Other than that, see if you can get the marks aligned with a new timing belt on and see if you can rotate the engine with the valve cover off and verify everything is moving properly while you or someone else rotates the engine with a ratchet and socket, NOT THE STARTER.