Rules that potentially involve the Internet have a few possible characteristics:
- Destination of Any (or All_Internet)
- Destination of Internet
- Destination involving an external Security Zone
- Destination involving an IP that is not RFC-1918
I presume you can write a script to parse the output of show-rulebase and pull out the rules in a specific policy layer that meet these requirements.
If you use show-as-ranges true in the call, you'll get the IP addresses covered by the rule, which should help in identifying such rules.