A sight guage can't lie to you, but it can certainly mislead you if you don't know how to read it
A word of caution - a sight gauge
can lie!
If the gauge is of the flexible pipe design and the pipe takes a relatively tortuous route from the tank, then trapped air bubbles can form giving a false reading. This is particularly applicable to aerobatic aircraft such as the Pitts. I am aware of at least one engine stoppage resulting from the pilot believing he had more fuel than he actually had for exactly this reason.