as mentioned above the easiest method that I know of is to select all of the ballons and open the edit/override menu which will give you an idea of how many were overwritten. The problem is that this doesn't show you specifically which balloos were overridden, but it will allow you to change only the overridden ones back to default.
Easiest method to select all balloons:
Hold Shift then make a right-click to open the selection menu.
Choose "Edit Select Filters"
Choose "Select None" and then tick only the balloons radio button.
Then simply drag a selection box across your entire sheet. (you'll need to do this on every sheet)
Checking the balloons:
As mentioned before, once the balloons are selected you can double click(assuming you're on IV2016) to open the edit/override box
This will now give you a list of all balloons on the sheet.
If the balloon is overwritten, the number under "override" will be highlighted in white.
If the balloon is default, the override box will be grey.
Again, clicking anywhere in this box will not highlight the balloon in question though, but feel free to switch all of these back to the default balloon value.