polarity check

Hi everyone, I have a Bissell rechargeable floor sweeper and have misplaced charger, I have a universal ac/dc charger that you select your output voltage that I can use as temp, I have to plug the correct connector on the end and can have pos or neg polarity on the centre tip, problem is I don't know which polarity the sweeper centre pin is and don't want to damage by connecting wrong polarity, help please.