Is it likely, could it have happened, I spose it could have. There's only one way to determine that, look at the valves,course thats the operation that doesn't appeal to you, nor would it to me.
Look at it this way, if seeing the valves only meant 4 screws and pulling the valve cover you'd do it to check the valves, right
It's all that work pulling the head that keeps one from taking a look at the valves.
I'd adjust the valves, see how it runs out and if it runs good keep on truckin.
Or you could do a compression test or a cyl.leak down test to access the condition of your motor.
Dan
