By turning on "Custom Vibrations" in the iPhone accessibility menu you can create custom vibration patterns as a form of caller ID. This way you can tell who is calling you without taking your phone out of your pocket.
This trick only takes a moment to setup. All you do is go to your iPhone's Settings and turn on "Custom Vibrations" under General - Accessibility. Once this feature is turned on you can setup a custom vibration pattern for that caller by viewing them in your contacts list and tapping edit. From now on there will be an option in the editing view to assign a custom vibration. It's that easy and very useful, especially when you phone starts ringing and its not appropriate to check who is calling.