parse it like this:

> 「いつになったら結婚するのか」**も** 聞いといてね
> 
> Oh, and **also** ask (her) when is she planning to get married.

**Edit**: for `のか` used as ending of question sentences, check [this answer][1]

P.S. `聞いといて` is colloquial for `聞いておいて`. I think we have other questions on the `〜ておく` form but I don’t have a link handy. 


  [1]: https://japanese.stackexchange.com/a/18994/3295