How do I know if my fuel sender unit is bad? See additional details.

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My fuel gauge needle is sporadic. It does not always reflect the actual amount of fuel in my fuel tank. Usually, after I turn the vehicle off, then on, several times and drive it, it settles down. This said, the range function on my Trip Computer is always spot on -- accurately displaying a mileage consistent with known fuel onboard. The range number does not vary with the fluctuating fuel gauge needle. Any insights and technical information would be most appreciated.