This is documented here for the Ramps board Why is my FarmBot not moving? however with the Farmduinos new power connector I can’t put the black reading pin of my multimeter on the screw of the negative Farmduino power cable in order to get a reading.
You could access the ground at each stepper driver, one corner (believe its top right) should be labelled with GND. Generally you can use pins labelled with GND, but often they are really near to other pins that could lead voltage and cause bad effects when you slip off with your probe.
For me it looks easier to use the ground of your peripheral output plugs as seen in the picture below. Only the right pin should be connected to ground directly. But be careful to not bend any pins in there!
Update: Using the housing of the USB port, or even the mounting holes of the board seems to be much safer in terms of bending pins.
Source: Farmduino pcb layout