There are Global Fields and Global Variables. There are no definitions for global variables. The variable get be anything and have any value. $$variable mean global $variable is a regular variable. Depending where the variable is used, such as a custom function then a $ symbol is not required. This variable would be local to the custom function.
You will have to read the script and see what the value is set to. If you have FMPA you can use the Data viewer to see what the script set the variable to. http://www.filemaker.com/help/11/fmp/html/create_db.8.33.html
Figuring what global variables are used in a FileMaker solution can be like looking for Easter eggs--both the organic and electronic varieties.
They can be hidden darn near anywhere. Not only can scripts create and destroy variables. (a variable with a null value does not appear in the data viewer), but calculations using the Let function can do the same. Such calculations might be inside a script step, a script parameter, an auto-enter calculation, a calculation field's expression, a custom format expression or a Hide Object when expression.
If you really wanted to compile a list of global variables, you'd probably need to use a third party produced design reporting tool such as Base Elements where you can do a find for specific text such as the $$ and see what you get--even then this would be far from quick or simple.
For an example of how calculations can assign values to global variables, see this demo file for a Date Picker placed inside a slide control that is in turn placed inside a popover. It's intended to replace the "spinning wheels" date picker that is standard for iOS devices to produce in FM Go something similar to the drop down calendar feature you can use in FM Pro. There are over 30 global variables inside a single panel of the slide control that are updated to display a typical calendar grid for a specified month and year. A script selects the month when the popover is opened and a "Hide Object When" calculation uses a Let function to assign values to all the other variables from that specified month and year.