connector - How to tell polarity expected of a DC barrel jack?

I'm sure this information is somewhere around the internet, but I can't find it.

I have a device that takes a DC barrel jack. I don't know the polarity it expects though. There is a marking though. It looks like this:

___
---  12VDC

I'm 99% sure it's a symbol to indicate the polarity, but which is it? Positive inside or positive outside?