The way you have it described, BigFix is not suited built to work quite that way.
The BES Client on the Target computers will be the ones running the Action, and they don’t have the ability to access the file on a Server Share because the Agent runs as Local System, thus has no user rights on a remote computer.
You could keep the file behind an httpd/WWW server, and have an action configured to become relevant every 12 hours, have the action directly download the file and put it where you want it on the target systems. At that point, why bother checking to see if it has changed, just replace the file every 12 hours (or what ever interval you want).
If you really want to ONLY replace the file if it has changed, compare the SHA1 Hash Values of the files.