If you own an iPhone 5s, give this a try. In the Settings app:
General > Touch ID & Passcode > Touch ID
You’ll see a list of your previously scanned fingers (Finger 1, Finger 2, etc.) Now place one of your scanned fingers on the home button. Touch ID will scan your finger and attempt to find a match. If it does, it will highlight that finger in its list.
This might seem frivolous, but it’s useful for verifying the fingers you’ve scanned. A nice touch, Apple.
[Via The Typist]
UPDATE: Loved this comment from BC2009:
A MUCH MUCH more important hidden feature of this screen is that you can use this screen to over-train your iPhone 5s with your fingerprints. If you have larger fingerprints you will find that after normal training the iPhone 5s may only have part of your finger registered (e.g.: the very tip versus the heel versus the sides).
However, from this screen you can keep tapping your finger to find the parts it recognizes and the parts it does not. Move your finger so it recognizes it and then slightly move your finger so that a part it previously did not recognize is also included. The iPhone 5s will learn those additional parts of the same finger. Once recognizes the additional region, you can move it a little more to include a part not previously recognized. Eventually, the entire surface of your fingerprint from the heel of the fingerprint, to the tip to the sides will be registered.
I’ve done this with my four registered fingers and it no longer matters which part of my finger I use and from which angle I use it.