One of the easiest Aircraft to visually inspect the fuel quantity in is the PA28 however unless it's full or at the tabs I'll use the dipstick to accuratly tell me how much fuel is in there. We never trust the gauges on GA types.
On the A330 , yes we have to trust the gauges and fuel uplift calculations to ensure the fuel is correct....
If you have a dipstick then use it when you cannot see precisely how much is there.
It's not rocket science.