Defective iPhone 5/ How do I know if my iphone is still under warranty?

Basically, just a few days ago I started experiencing a problem with my Sleep/Wake button and I noticed that it only works when pressed on the far left side. If I were to go to the Apple Store to try and it replaced, how can I tell if my iPhone is still under warranty and if they will replace it before driving all the way over to the Apple Store? (which is about 2 hours away from me.)