Yeah but that effect is stated in the CMI, the CMI recommends you don't even bother attempting to try and get pregnant for 3 months after stopping the pill, and for those people who were irregular before starting it can take up to a year. But also realisticly, there are like 6 days? In a girls cycle when she can get pregnant, if you're not doing it then, or you've been doing every single day multiple times up until then, your chance of getting pregnant is lessened.
As for telling whether a girl is on the pill or not, I work at a pharmacy, which serves most of the people I know, if it wasn't for the fact I dispensed the girls' prescriptions, I'd have no clue they were on the pill.