I've always been a firm believer in "Listen to it enough and play what feels natural and it will be close to, if not exactly, correct." Just play the song a few times strumming whatever comes out naturally. If you've heard the song a lot, your mind will probably get you pretty close to right.

Now this is just my opinion!! It works for me, but I know it doesn't work for a lot of people! It's just something for you to try. It's never failed for me.