How would I check if the polarity is right, do I need a multi-meter? This adapter doesn't have a ground terminal and both the positive and negative terminals are the same size(width wise), so it can be plugged in two different ways.
Most of the USB hubs that I have come across are 5V POSITIVE polarity, i.e. the center tip is positive.
If you are using a multimeter, place the positive lead (usually red color) on the center pin of the DC input. Place the ground lead (usually black in color) on any metallic surface of the hub. If the reading shows positive value, then the DC input is POSITIVE polarity.