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How do you determine DC polarity when the device does not specify?

My device is the ASRock Vision 3d Series (https://www.asrock.com/nettop/intel/Vision%203D%20Series/).
It does not have anything denoting polarity on the outside of the device itself. And from what I can find, it does not tell you on the product page or the manual either.
Link to the manual: https://download.asrock.com/Manual/Vision%203D%20Series.pdf. I don’t have the original power unit.
I started taking the device apart to look for something that would tell me, but it looks a bit daunting.
If nobody can help me determine the polarity from what I have provided, what should I look for when taking the device apart? Do I try and find the port from the inside of the device? Do I look somewhere else?