I want to use a USB power bank for powering my RPi3B but I need to display how much charge is left. What is a good set up of measuring the battery level of the power bank?
USB power banks vary, but mostly they will have a single 3.7V cell and a boost converter to bring that up to 5V. The cell will be charged to, say, 4.2V and decrease to, say, 3.1V. The boost converter takes this varying voltage and boosts it up to 5V for use by USB devices.
The power level is a function of the voltage of the cell. The output voltage will stay the same regardless until the cell is discharged and it shuts off. Unless you take the power bank apart you won't be able to measure the cell voltage from the output.
Some power banks do have a few LEDs to indicate level, so you might be able to interface with those, or if you want something more accurate use an ADC (analog to digital converter) to measure the cell voltage. If you've gone that far, you might be better off just using the cell and finding something like a lipo shim rather than designing your own circuit (I don't know off-hand of any that do give more than a simple low battery warning).