The best way to sex your tarantula is from its cast skin. This is best done if you remove the shed skin as soon as possible after the tarantula as finished moulting, then the skin will still be pliable and easier to work with. If the old skin as gone brittle it can be softened with water and a small amount of soap. A magnifying glass or microscope are  handy tool to have,especially for the smaller specimens.

You are looking for the epigastric furrow which is between the forward booklugs which are the white circular parts of the pictures. The epigastric furrow looks like a line between the front booklugs. The female tarantula as a leaf like structure that is called the spermathecae on the inside of the epigastric furrow, this is where the female spider recieves the males sperm after mating. Males have no spermathecae . So it is the presence or absence of the spermathecae that tells you if your tarantula is male or female.

The pictures below show a FEMALE Brachypelma smithi. You can clearly see the spermethecae above the epigastric furrow.

The pictures below show a MALE Brachypelma bohemei. Note the absence of the spermethecae above the epigastric furrow.