I know this isn't what you want to hear, but there is only one way to figure that out. You have to practice. There is no secret method for it. You have to play and listen to a lot of songs, and eventually something will start to click and you'll find it easier and easier to hear how the strumming patterns go in songs. Timing, upstrokes and downstrokes, you'll just hear it, you'll just know. Practice, practice, practice! That's all you can do! Good luck.