You really can't unless you've talked about it a lot before. Just say "how are things going?" in a general sense and if she wants to talk about it, she will. Or you could say "are you guys still thinking about kids" and see what she says. But if she isn't bringing up the details on her own, she probably doesn't want to talk about it.
My sister and I are pretty open about this stuff, and I take credit for helping her conceive her second by introducing her to OPKs -- worked for her on the first try.

(I have, unfortunately, a ton of experience with OPKs over the course of 5 pregnancies and 2 kids.) But a lot of people aren't. I have friends who I know are struggling and I would love to share everything I've learned through my own struggle, but I don't think they'd be open to it, so I haven't.