how to get the edit box value is changed run time to change visibility of other field on same page?

how to get the edit box value is changed run time to change visibility of other field on same page?

Both the field are on same level. I do not want to press tab after entering value in edit box.

I have a edit box as a comment.  It is a regular field.

When I come on this page , this is the only available field that I can change. Once I make any change in comment edit box there is another list box(with yes/no value) that should be available and should be required to answer.

I do not want to make the user to mandate to press tab after making change in comment