My guess is that black will be for the signal return, red is to be connected to the supply output, and the white will be for the analog input. But you should definitely check with a multimeter. [ Edit: and indeed my guess was wrong. ]... the PB-8 has three wires and they are white, red and black which finally brings me to my question...
How do I tell which wires on the PB-8 are the supply output, throttle input and signal return?
A cheap one from an automotive store will do for this purpose, but if you don't yet have a decent multimeter, perhaps now is the time to invest in one.
The pot box is a resistor, usually 5 kΩ (kilo-ohms). So two of the leads should show that same fixed resistance no matter what the pedal position. The other connection is to a "wiper", that can connect to one or other end, or anywhere in between, depending on the throttle position. So at rest position, I expect to see little resistance (perhaps 100Ω maximum) between two of the wires (white and black are my guesses), and that will increase as the pedal is pressed until at maximum pedal depression it should equal nearly the full resistance of the "pot" (potentiometer). At full depression, there should be little to no resistance (a few hundred ohms maximum) between the wiper and the end that you will connect to the supply output.
If you get this wrong, you could have your controller provide full power to the motor at switch-on, which of course could be dangerous. Hopefully, your controller will detect this and assert a "pedal stuck" error of some such instead of applying full power, but the simpler and/or cheaper ones may not. Best to be sure. There is also the chance that you could short the supply output and damage the controller. Again, unlikely, but again, more likely in the simpler and/or cheaper controllers.
Hopefully, you'll know how to use a multimeter to measure resistance. If not, you can find it online, and this is a good skill to have when dealing with EVs (including boats, of course).