The only way to truly know how your good your fuel guage works is to run the boat so it runs out--once. Most tanks will still have some fuel in them, but not enough to get to the pick up.
Run it out close to home or close to somewhere where you can get fuel and have 5 gallons with you to make it to the pump. My guages have been fairly accurate. I can tell you, when the get on E and the needle is not bouncing, you're on borrowed time.