Chords

When you see a song and it has the chords to play, how do you know the strum pattern if it isn't stated?

You don't. You just dive in and make one up.

try listening to the actual song and go from there

I totally agree with da guys here ... if you know the tune/melody and have a feeling/rhythm of the song, come up with you own strum. IMO, far too many folks try to emulate the original style (I for one). Although, some songs do need a certain strum to make it sound right, it's okay to use a different strum.