Unless someone's been in there bending the tang of the floats about there shouldn't be any adjustment to do. Normally the problem will be the valve itself leaking, either because of debris or damage to the rubber cone, or failure of the o-ring that seals the brass valve to the body of the carb. The other common failure is the float leaking, filling up with fuel and sinking.
I've not found a better way of determining the fuel level than the clear tube method, though I did have difficulty getting sensible readings.