Specific character check in window

[layman]

I need to run a "checking". If two of my items are checked, I need to uncheck the previous one. So, all I need to do is to get the "character" (check in a check box) in two specific places (pixel) of my window and test whether they match or not. Now, how to do that? What is the function involved? I tried to check the color, but that isn't helping much since the pixel position cannot be specified that precisely under my situation.