Back in my gas pump jockey days, we used to stick the tanks daily with a wood stick when we read out the pumps. The amount pumped need to roughly match what was used up from the tank that day.
Besides checking the depth of the fuel, we also checked for water - we smeared a special compound on the lower 4 or 5 inches of the stick. It would change color if it found any water. There was a certain amount that was acceptable, but I can’t remember what it was. Condensation inside the tank, settling to the bottom because gas is slightly less dense than water. Didn’t get to the customer’s cars because the pickup floated near the surface of the fuel, not at the bottom of the tank.
If the water content was too high, we’d run it down, then pump out the tank and refill (had to have someone come and do that - we gas pump jockeys didn’t have that skill nor the equipment).
Sticking the tank also measured the depth of the fuel vs what it should be - that is, we’d compare the amount in the tank versus what was supposed to be in the tank. We’d also stick them immediately before and after a delivery to make sure we weren’t getting shorted. We had charts of inches vs gallons, taking into account the shape of the tank, of course.
Oh yeah, I was told back then that wood was used so as to not be a potential source of static electricity discharge - a spark at the opening of a tank full of gasoline might not end well. Wood is a really bad conductor so it’s ideal for the task. And cheap.