Do a custom editor script that sets up an int for each sprite in your atlas and run through all your widgets and add up for each that use that sprite. If any is 0 at the end, you can (carefully) remove the sprite from the atlas if you want.
Do note that if you set them by code, this won't work, so you'll have to get more clever and do it in playmode and go through all (ALL) your screens and do the same.