Oh. Well sometimes you're able to tell by looking at them. Take Meganium for instance. Meganium have antennae on their head. The antennae on a male are longer than the ones on a female. Female Pikachu have notched tails.

For those that don't have a physical difference like that, your Pokegear should be able to tell you.

Some, however, are genderless. Like Porygon or Golett.