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.