[350HP930]

Yes, the valve spring is going to be your biggest enemy when trying to figure out what kind of shape your valve guides are in.

Removing the rocker and spring with the engine in the car is an option, but one that would require a lot of work.

I know one rule of thumb is that if you properly adjust the valves but still have noticible intermittent valvetrain noise that is an indicator of worn valve guides.

Another is that most air cooled engines will be due for a valve job between 80K and 120K even if it is treated well.