Boeing uses underwing float sticks on most models (excluding out-of-production with dripsticks, and 787, which does not have a mechanical fuel indication system. Don't know about 777x). The work and look like as shown below. If you Google for "fuel measuring stick manual" you might find something interesting to read.
https://en.wikipedia.org/wiki/Floatstick
Fuel Floatstick/Dripstick