Calibrating steps per mm/in on A,B,C steppers on a delta

I've got a delta printer, and I've got Marlin 1.18 working on it. Seems to be okay, but I've printed a calibration cube with it and it's a little off (20x20mm comes out to about 20.0x19.6mm).

I suspect that my steps-per-mm on the carriages might be off (most likely due to some stretching of the belt). What I'd like to do is tell Marlin to traverse one of the steppers by a certain distance or steps and then measure how far that actually is. Is there a way to get Marlin to do that?

You can tell it to just move in Z and then all 3 axis will move the set amount and you can measure any/all rails.

#facepalm
Wow. Now I feel like an idiot.