Although if you're using a variable $var in the script it isn't necessarily the same as $GLOBALS['var']. But then in any case you could just read the function and see. If it's declared as global in the function then it's global; if it's an element of $GLOBALS[] as devinemke noted, then it's global. If it's an element of $POST, $GET, $_SERVER, etc., then it's global. Otherwise it's not.
It's possible to write code that might declare a variable to be global only if certain conditions are met, but that seems like a seriously bizarre thing to do.
