You'd select all the files on one side and then see which ones are selected in the other side afterwards. No need to think about which files to select.
You could make the button do the select-all for you. You can also make the button highlight the filenames on both sides which exist on the other, like this:
Select ALL
Select SOURCETODEST DESELECTNOMATCH
Select DESTTOSOURCE DESELECTNOMATCH
(Taken from this thread: Does not exist )