Agreed on having to pull the head to get a good look at everything. You can actually swap out the retainers without pulling the head by using a leak-down or compression test hose attached to compressed air, although it might take rigging something up for the spark plug since the cam cover is off. The air keeps the valve from dropping into the cylinder.I'm not sure how you would check the top of a piston on any engine without pulling the head. Some kind of TV camera, I suppose, but my guess is finding somebody to do it would be pretty tough. Pulling the head is going to be necessary to put the valve back together anyway, unless somebody knows something I don't know.