It's easy. The male will have a little penis that is much further forward. When he lays on his back, it almost looks like it is on his tummy.

A little bitch will have the vulva which is much further back nearer to her anal opening and easier to see from the back if you lift her tail. The vulva is sort of pointed at the end and wider at the base and has a slit running down it.

The penis on a puppy often has some hairs growing from the end of it.

Hope this helps.