Quickest and most simple check is to ensure that your black wires simple run parallel with your red wires throughout the connector (example below - note picture below is for illustration only to show how the wires should look if correctly connected - parts pictured below are not for sale in this listing).

If for some reason your red wire runs through the connector with a black wire connecting where the red wire should be on the other side then you will have an issue with polarity. Dont use the connector or risk damaging your equipment (example below - note picture below is for illustration only to show how the wires should look if incorrectly connected - parts pictured below are not for sale in this listing).

In most cases: Black wire will be negative. Red wire will be positive