Samsung Adapter 25W

Not sure how effective this is but if you connect your phone to it, the charging animation will either be green or blue. Blue only shows up on official chargers. If your phone doesn't support super fast charging I'm unsure if this works

Besides that, you could try looking up the serial number of it

Where did you buy it from? Reputable sellers wouldn't put a fake charger and sell as original. Also read the small print on the box and on the charger itself. Compare with your other chargers and see if it's same.