Well its simple. It will be in the in the top right corner of the screen. Its a rectangle with a line of 5 "+" signs going from the top left to the bottom right or vice verse. Actually enhanced call audio is turned on, and the icon is an equalizer. Therefore, you have bass boost or treble boost turned on. The treble boost is in opposite direction of bass boost, and would be off for normal.
You can change this option either while on a call by pressing the menu key and then Enhance Call Audio, or you can go into your call history (press the call button from the home screen), press the menu key, select Options, and then General Options. The enhance call audio options are at the bottom of the list.
Blackberry Support Engineer