Well it's easy to know who's what if you grew up with them. And of course my exmuslim friends know that I'm one, they don't come up to me and tell me because they think I'm a pious Muslim ffs, and the rest think I'm an irreligious Muslim. It's always been other people who've asked me if I'm an exmuslim, and if I trust them/know them well enough I say yes. And in almost all cases I already had my suspicions about them as well. Easiest way to meet exmuslims in Pakistan is through dankmeme pages outside of just private schools. Being in Islamabad I'm sure you must've heard of headstart, the exmuslim Mecca, literally half the kids I knew were openly atheist.