There's no way you could know that.
Yes, there is, actualy!
ZA logs all hack/probe attempts. While I was using bittorrent software with ZA (frequently), I had several "high danger rated" probes intercepted by my ZA firewall. Using the other firewalls, I basicaly had to turn them off to get bittorrent to work properly. So there, it did in deed save my PC from trouble. I also visited some sites while using BT that had tons of spyware, and ZA blocked it.
It MAY have been possible to config ports, etc with those other firewalls (I still don't know as they were so complicated). But I, like most computer users, are 'newbie' types when it comes to advanced or expert configuring, so I would have ended up probably running no firewall at all. I realize that is hard for an expert like you to understand, but thats the reality. Of the dozen or so of my friends that have computers, only 2 would even know what a port is...
So anyway, I found ZoneAlarm really easy to setup and run, and it gave me zero problems for a year and a half with no viruses or other junk. TNG out.