Saber is knowing how to play the guitar, knowing how to do something, knowing a language, having knowledge about something.
Conocer is more like being aware of something, not necessarily having knowledge about it. If you say "conozco la guitarra" it means you know the instrument, you know what a guitar is, but it doesn't imply you can play it. You use conocer in the context of knowing a place, knowing a person, and so on.
So you would say "¿tú sabes tocar la guitarra?"