Actually, there's a lot more to *acurately* sexing than just looking for a "flap of skin" because, in some species, especially when young, males sometimes appear to have "flaps" also.
The revelant features of a theraphosid to look for in a shed exuvia are:
Arthordial membrane; shed muscle attachments that mark the limits of the epigiynum area on the left and right.
Slit sensillum; small button hole like stress detectors that can be found in the upper alcove.
Upper alcove; forms the nose of a females epigiynum area and is where you will find the females spermatheca.
Spermatheca; found only in female theraphosids, this is the place where a males sperm is stored after mating. It develops from a paired structure but can form into pared or fused organs and is located in the lower half of the upper alcove behind the uterus externus that it extends off from.
Uterus externus; a thin often transparent pocket like membrane found only in females, the opening of which is called the gonoslit. It is the only part of the uterus that is shed during a moult and joins to the uterus internus and leads to the oviduct and ovaries, but these of course are retained by the tarantula during a moult..
Gonoslit; this is the opening to the females sexual organs that can be found in the epigastric furrow.
Epigastric furrow; this is located on the underside of the abdomen between the two anterior book lungs. It is a fold in the skin that’s leads to the gonoslit or in the male’s case, the gonopore.
Bursa copulatrix; often called the lower alcove; this is where it is said eggs and sperm mix during egg laying.
Antererior book lungs; these are the two front most feathery type lungs nearest the cephalothorax that the spider uses to breath.
Gonopore; this is the opening to the males sexual organs. In a moult it appears small and buttonhole like with thick lips.
Male epigastric organs; found only in males these organs produce a liquid that mixes with the sperm (recently found out they may also/or be slilk glands use in sperm web production). They can be mistaken for spermatheca especially in smaller moults if you’re unsure what they look like.
In some species, females do *not* have spermathcae; so.... how does one sex then if there is no "flap"?
You look for the uterus externus.
Because male epigastric organs can sometimes be mistaken for female spermathcae, it is often wiser to look for the uterus externus to rule in or out whether the spider is female.
This is especially important when attempting to sex a spider that is only at 5th or 6th instar and thus, still small and not completely developed.

*Note*
This spider is most obviously well past 5th-6th instar (meaning it has probably molted more than 6 to 7 times) and indeed *does* show a clear uterus externus. If Snakeman wants, I'll take that picture (or rather an unedited one without the circle around it if he wants to provide it) and point out the various things to specfically look for so everyone can see what I'm talking about... and so all of my yapping at the beginning of this post will make more sense.

Post a pic of that girl once she's done stretching man. Genics are beautiful!!!!