The other, which I used, is to pull a branch(? I don't know what to call it in English) in the middle of petals. After you pulled that branch, the rind will be easily crack down and then you can see white, soft, sweat & sour flesh.
Not only its flesh is tasty, containing calcium, phosphorus, iron and some vitamins (that what I heard of, but not confirmed), but its rind is also used to heal bruises and wounds, fresh or blister. Mangosteen's rind has substances called Tannin, Xanthone, and Mangostin. Tannin help a wound to heal faster, Xanthone acts as an anti-fungus substance to prevent skin disease and ringworm, and Mangostin is anti-bacteria substance. It also used in cosmetic such as acne soap and facial wash. Anyway, I don't care how many benefits it has, the best part is the white sweat flesh. And by the way, don't eat its seeds, it's not edible.