how to diagnose valve float?

are other ways to diagnose or should i use a vacuum gauge?

first thing to check would be your valve spring pressure. Generally, if your valves are floating you'll get backfires, through the tailpipe and the carb, and usually some broken parts when valves start pounding on pistons....

good time for a lot of us to learn something .. i had associated " valve float " with weak springs .. hence not closing the valve quick enuff .. can that also cause " valve bounce ".. and also lifter bleed down at hi speed .. does that also give valve float or bounce symptoms .. hence the old bragging phrase '' " iv`e got anti pump up lifters "'' what do anti pump up lifters cure ??

I second the recommendation to check valve spring pressure. You should or could also feel a loss in power during acceleration at a certain rpm. Of course, by then you're entering into the danger territory.

I had a lifter going flat. It was like an intake leak. Didn't notice it 'till I pulled the valve cover when it was running. You could see it plain as day. Would image valve float would have the same symptom under light loads.

I would like to add everytime you "float" your springs it will accur earlier the next time you decide to rev. the engine up.