Assuming a Lofrans Cayman, the three electric terminals on the windlass motor work like you described. There's a common negative and two positives. One for up, one for down. The control box (which is just a big two pole relais) switches the battery positive to either one of those poles.
So when you put a multimeter between negative and UP positive, you should see a voltage (probably about 10-11V depending on your cabling and windlass load) while the UP button is pushed. And vice versa for DOWN.
There's a wiring diagram for all of this in the manual. Beware, the manual covers multiple models, so make sure you look at the right diagram (for Cayman, not Atlas).