The only thing I can think of would be to run a database design report for just your scripts.
Then run a find in on the report for $$ it should show you all your global variables and tell you how many it found..
There are a number of third party developer tools that work from the XML version of a Database Design report to provide value added features. I wonder if any of those would provide the ability to produce such a list of global variables?
The fact that global variables can't be easily tracked like this is one reason I recommend only using a global variable under those circumstances where you have to use a global variable. Sometimes a $Local variable is all you really need and you can pass the contents of a local variable to a different script by including the variable in a script parameter--this is even possible when you need to pass the values of multiple variables to the same script. Thus, you can use "scoping" to keep more of such data in $local variables that automatically cease to exist when the script exits.