Would suggest to connect a PC on the public facing interface and run some test from that point of view. Nmap is a great tool for port scanning and fingerprinting and you won't be limited by "slow" internet bandwidth. Of course you can go with more detailed and structured firewall test tools like AlgoSec’s Firewall Analyzer and Nessus.
Watch the log files generated on the firewall during the tests and consider issuing specific attacks to targeted "published services". Is a common security practise to install a network analyzer to a potential "victim" in order to check that all packet fragments are blocked by perimeter devices.