cracks knuckles here we go, omegaverse anatomy 102

So generally, there is multiple possibilities for this. Sometimes in omegaverse, that is true: your primary sexual organs CAN tell what subgender you will present as. E.g. an alpha is born with the dick and balls package, an omega with the pussy package, a beta is either of those based on whether they are male/female. So, yeah, TECHNICALLY Megumi should have been aware he couldn't be a beta even before he presented. Since he's a male omega and all and male omegas as well as female alphas would be the exception then.

BUT
sometimes in omegaverse betas can pack EVERYTHING, so a male beta could also have a 🐈 though it might be rare. That's the reasoning I used for Cranberry Juice but it is never really explained (maybe Megumi was just delusional and had hope lol)

In a a/b/on universe where omegas have dicks and we have the self lubricating butthole thingy, presentation definitely makes a LOT more sense because then you only know what you are if you start slicking or forming a knot lol. But I like omega's with pussies, so I went that route🤷

For the primary gender assignment at birth o think in omegaverse, at least in the second and last scenario, you would only be able to look at chromosomes🤔 Which is kinda shit, considering we know how there is many ways this can go nowadays and that it doesn't always mean the correct gender is assigned at birth. Anyway, the omegaverse is a mess and I try not to think toooo much about logic😅