You've just experienced one of the big problems with using global variables in LabVIEW. If you use global variables you should always try to write accessor functions that read/write that global variable so that you limit access to the global variable. In OO-speak, make the gloabl variable a class. Sort of.
Any way, one way you can do it is to open your top-level VI, then open your global variable. Right-click on the icon pane in the top-right of the global variable's front panel and select "Find all instances". You can then see what part of the global variable is being used. Keep doing this until you've checked off the elements that are being used.
Another way is the brute force method: open your top-level VI and then open your global variable. Then, start deleting elements from the global variable. If the top-level VI breaks, then you know that element is being used. If not, then you know it's not being used.
