How do you sex a tarantula by looking for epiandrous fusillae? Look through the various pictures in this section. In particular, look for pics of tarantulas judged to be male. Look just above the epigastric furrow and you will see an elliptical or triangular symmetric patch, usually darker, that is centered from side to side.
That dark patch is the epiandrous fusillae. They are silk producing glands that only males have. They are for the specific purpose of creating waterproof silk to line the sperm tent with.
The fusillae becomes apparent at different ages for different species of tarantulas and the shape and size vary as well. It can be as misleading in some instances as spermathecae but at least you can attempt this method at any time.
The more examples you see, the easier it will be for you.