Jeff Gamet at The Mac Observer posted a very helpful article today on how to identify which of your 10 fingers are linked to your Touch ID on your new iPhone 5s. He walks through the six steps necessary to differentiate which of the saved five fingers you use as your passcode. (You did use a finger, didn't you?)
One caveat to this post though: You can "assign custom names to each fingerprint" by opening the Fingerprints screen, then tapping edit and tapping the name, which is listed as Finger 1 through Finger 5. Choose the one you want to rename and type away. This part of the tip is offered by The Mac Observer reader Nik Jones and tweeted by @mcelhern (Kirk McElhern). Thanks guys!
If you need help setting up the Touch ID on your new iPhone 5s, Apple support has just the document you can read, iPhone 5s: Using Touch ID.