Ignore the spots and look underneath
The markings of a young Appaloosa tell you nothing about its sex, so put the coat pattern aside and check the foal's underside instead. With the foal standing calmly beside its dam, lift the tail. A young filly shows two openings close together directly under the tail: the anus, and below it the vulva, a short vertical slit. A young colt shows only the anus under the tail, because his external genitalia sit much further forward, midway along the belly, where you will find the sheath. That single difference, two openings under the tail versus one, is the clearest answer at this age.
Checking a young Appaloosa colt
Run a hand gently along the colt's belly as he stands, and you will feel and see the sheath, a small fold of skin pointing the genitalia backward, roughly halfway between the front and hind legs. Do not expect to confirm a colt by finding testicles in a very young Appaloosa. In many foals the testicles have not yet descended into the scrotum, or they sit high and small enough to miss, so an empty-feeling scrotum area proves nothing either way. Judging by the sheath's presence and the single opening under the tail is far more dependable than hunting for testicles that may arrive weeks later.
Checking a young Appaloosa filly
A filly confirms herself from two directions. Under the tail you have the vulva sitting just below the anus, and between her hind legs, on the underside where an udder will one day develop, you can find two small teats. Colts do not show those teats in the same way, so the combination of the second opening under the tail and the small teats between the hind legs settles the question. Appaloosa fillies and colts can look equally bold, equally chunky and equally spotted, which is exactly why build, behaviour and coat should never be part of how you decide.
Handling a foal while you check
Work with the dam close by and, ideally, a second person holding her, because an Appaloosa mare can be protective and a foal that has barely been handled will move at the wrong moment. Crouch rather than kneel so you can step away, keep one hand resting on the foal's hindquarter as you lift the tail, and make the whole check part of a short, calm handling session with stroking and a calm voice rather than a grab. Foals wriggle, and a half-glimpsed check leads to a wrong record that follows the horse, so if you did not see clearly, wait for a quieter moment and look again rather than guessing.
Getting it confirmed and recorded
Ask the vet to confirm the sex at the Appaloosa foal's first check, when they are examining the foal anyway, and record it the same day alongside the foal's colour, markings and date of birth as you know them. Getting it right matters more than it sounds: feeding groups, turnout companions, future handling and every later record build on whether this foal is a colt or a filly, and a mistake discovered at a year old unwinds a year of paperwork. Note it in the foal's own records straight away, not in a message thread. KinsLe's Pet Pass holds a young Appaloosa's identity and owner contact details on its verified ID card, so the foal's sex and markings are shown correctly from its very first record.