This depends on what she is asking you. Listen for any hints, but don't push her. Casually drop marriage into the conversation and listen carefully to her views.
If you've known the lady for a long time and if you are ready, do this: Buy an engagement ring, with a diamond or a transparent zircon (the look-alike-diamond) make it simple meet her somewhere or at...
Play hard to get? Like, if he thinks he has you there's no need for him to do it. He needs to see how much he needs you in his life and appreciates and loves you. Maybe take yourself off on a holiday...