How can one detect airplane mode on Android?

By extending Alex's answer to include SDK version checking we have:

/**
 * Gets the state of Airplane Mode.
 * 
 * @param context
 * @return true if enabled.
 */
@SuppressWarnings("deprecation")
@TargetApi(Build.VERSION_CODES.JELLY_BEAN_MR1)
public static boolean isAirplaneModeOn(Context context) {        
    if (Build.VERSION.SDK_INT < Build.VERSION_CODES.JELLY_BEAN_MR1) {
        return Settings.System.getInt(context.getContentResolver(), 
                Settings.System.AIRPLANE_MODE_ON, 0) != 0;          
    } else {
        return Settings.Global.getInt(context.getContentResolver(), 
                Settings.Global.AIRPLANE_MODE_ON, 0) != 0;
    }       
}

/**
* Gets the state of Airplane Mode.
* 
* @param context
* @return true if enabled.
*/
private static boolean isAirplaneModeOn(Context context) {

   return Settings.System.getInt(context.getContentResolver(),
           Settings.Global.AIRPLANE_MODE_ON, 0) != 0;

}