If there isn't any symbol for it (it usually is) I recommend contacting pinball life, they should have the data. Sorry I didn't have any better answer, hopefully someone who knows can check.

Check if there is a part number on the pcb and google it, moet of the times you can pull up the data sheet .Get a voltage meter and put it on beep. And get a negative on any traces . Most screw holes are negative also. And ping it . If it beeps you know the negative

9/10 times the middle male is +

Which honestly is by far the most common configuration.

For future reference though, you could probably use a continuity tester to check the polarity. Circuit boards generally have pretty obvious ground plane/ground traces, which should connect directly to the negative side of any power supply. You could confirm the center-positive polarity by checking continuity between the circuit board ground and the outer wall of the connector.

Granted, the PCB really should have been marked, but in lieu of that there are ways to figure it out. Asking others being a really good option, but if that had failed or if you wanted to get the answer sooner, it's useful to know other techniques.