Although you generally cannot determine the gender of a cat by color pattern, it is a safe bet to say that most calico cats, like tortoiseshells, are female. It is not impossible for calicos to be male, however, so if you have one, you're one of the lucky few!
You may never actually encounter a male calico cat in your lifetime, but unlike the Loch Ness monster, they do exist! For every 3,000 cats with the coat pattern, roughly one is a boy -- approximately .03 percent.
Calico cats, unlike say the Siamese, are not a breed but rather a genetic disorder of sorts. In the cat world, fur color is a gender-specific trait related to chromosomes. Female cats possess dual sets of X chromosomes, which means they are capable of showing combinations of orange and black -- the colors that are associated with the aforementioned chromosome. Males usually only have one set of the chromosome, so in the rare instance when a tom cat has two of them, he too can enjoy a beautiful calico coat.
Some might wonder what would happen if they bred a male and female calico together. However, this task is for all intents and purposes impossible. Since male calicos have a genetic abnormality, the majority of them are sterile and therefore unable to reproduce at all.
Another instance in which you can guess an adorable cat's gender based on its coat is the orange tabby. While most orange tabbies are male, however, female orange tabbies are significantly less rare than male calicos.