If the calyxes are directly attatched to the stem, it's a female. When they're separated by a tiny stem as an offshoot of nodal region, they're males.
Simply put, male calyxes are raised off of the node. Females stuck right in there with no secondary stem.
This varies from strain to strain, depending on the genetic quality of the seed, or on other factors including stress.
Once and a while a female can have raised calyxes, I can't really tell from the pic.