Orgasm? How does it work!?

I seriously don't know how to cum. And I always get embarrassed when me and my boyfriend are doing stuff, because he'll be going down on me for what feels like forever and he'll check up to see if I finished. And I have no idea, I always feel the sensation and it's not like he's bad at what he's doing it feels amazing, but I just don't know how to orgasm. But when we used to have sex I would be able to, but I wouldn't know it had happened. So if I feel the sensation what do I do? Like push? That sounds so weird but do I have to kind of push it out?? Plz help