You may not always be able to - but always be on the look out for clues about what frame of mind they are in and what they might want to talk about.
Also to prepare for if she's just completely bored, you should specifically remember things that you find interesting that you would like to talk about. Some things you saw on the news, something you read interesting recently. Even if you haven't conciously made the effort to remember some interesting things you saw, think back and see if you can remember any.
Failing that, talk about movies, ask what she's seen, what she likes, some of the content of the movie might remind you of something you want to talk about.
But all the while - get her to talk whenever possible

Look for opportunities to ask her what her thoughts are on the matter. Women love talking about themselves and their opinions.