Handling a tarantula always is problematic, and holding it upside down can be stressful to both you and the spider. Instead, many breeders prefer to determine sex, especially of females, by examining cast skins under a microscope.
The procedure is moderately complicated and requires both experience with a variety of species at different ages and a reference collection of shed skins, preferably of spiders followed through to their maturity molts (males) or to the laying of eggs (females). In basic terms, the skin is soaked in soapy water to make it flexible, and then the underside of the anterior abdomen is examined. Remember that you are looking at the skin from the inside out, preferably with a bright light shining through the skin toward the observer.
(At least a 10 x loupe is required, and a stereomicroscope is much better.) Find the four book lungs, which appear as two pairs of whitish squares. Between the generally smaller front pair will be the epigastric furrow, and in the center of this will be a pore that is the primary opening to the internal sexual organs. In a male, the furrow is simple and low, without projections.
In a female, even an immature one, there should be small tubes or pockets to the front of the furrow; these are the spermathecae (singular, spermatheca), which are sacs in which the female stores sperm inserted by the male to fertilize the eggs at a later date. As a rule, the tubes are narrow, are widely separated along the furrow, and end in rounded bulbs, but they also may end in a pair of bulbs (as in king baboons) or have very wide bases that together cover much of the length of the furrow and are much wider than the bulbs at their ends (Costa Rican striped-knee).
In most of the common Brachypelma species (including the red-knee and painted red-leg), the spermathecae are fused into a single oval pocket the width of the furrow and lack distinct external bulbs. Regardless of shape, the shed skin of a female tarantula more than about six months to a year old should show the spermathecae.Some tarantula clubs and experts offer to sex your tarantula from a shed skin. The price is low and the degree of accuracy is high, so if the service is available, utilize it.