Would the "AOA Disagree" light be the only way 737 MAX pilots could detect malfunctioning AoA sensors?
If there was an AOA gauge, then looking at that could have told him there was a problem without an AOA disagree light. 

It'd be better than the AOA disagree light too. If the same mechanic maintained both AOA probes or their signals were sent through the same wire harness bundle, they could both agree on the wrong answer.