When I'm looking for a particular iPhone feature for my iPhone app, how do I know what class or framework to look for in Xcode documentation?

When I'm looking for a particular iPhone feature for my iPhone/iPad app, how do I know what class or framework to look for in Xcode documentation?

The developer.apple.com provides the best documentation. For example the UIAccelerometer page at:

http://developer.apple.com/library/ios/#documentation/uikit/reference/UIAccelerometer_Class/Reference/UIAccelerometer.html

says at the top that it is part of the framework /System/Library/Frameworks/UIKit.framework Framework