How do you tell the difference between the back and front of the skateboard?

Generally from the materials. A fake skateboard (one from like Walmart or most sporting goods stores) is made from plastic or low quality wood. The wheels also may also be plastic, if it's like 50$ for a pre-built full skateboard, its likely a rip off.

Make sure to get a skateboard from a skateshop or online from a skate supplier or whatever (I.e. www.ccs.com, ecboardco.com ect..)