I’ve used inline USB meters, but they don’t allow for a poor down-stream cable. They also have an internal current shunt - ie.the display is either the PSU voltage or the voltage seen by the Pi (no idea which).
I picked up some $1 voltmeters and attached them with velcro to the Pi case.
Accuracy is quite acceptable, but is also adjustable (via a pot) should you feel you have a better reference.
I’ve jacked it into the GPIO port to measure what the Pi really has to play with.
The voltmeter module is a two-wire design, meaning it measures its own supply (usable from 2.6V ~ 30V)
(not ideal in a noisy or precision environment, but really easy to work with)