[Mart]

Pull the plug in that cylinder and you can observe the exhaust valve directly below the plug hole. If that one is sticking open you will be able to see it.
You could remove the intake. You can see the valves in there and might be able to manipulate them up and down.
Squirt some penetrating oil anywhere you can that might get around the valve stems. (down the port for instance).
If you take another movie hold the camera sideways. It comes up bigger on the screen.

Fwiw the only time I experienced sticky valves was in a Y block. I was trying to be clever and was burning some very old fuel. In that motor it manifested itself by bending the pushrod.

Mart.