Set the deck onto the ground and look for the curved nose and tail. The curve is where the wood of the deck flares up from the length of the skateboard deck.
Look for the narrower end of the skateboard. On long board style decks, a long and narrow tail indicates the back of the deck, with a flared and less-narrow nose.
Measure the front nose width at the start of the curve going from one end of the deck base at the curve start to the opposite end of the curve. Measure the back end at the curve. The longer curve is the back, with the shorter being the front.
Check the measurements for equal lengths. Decks like this are used with either end being the front or back.