A good diagnostic tool for any sysadmin is a port scanner to ensure a firewall is working as intended to open or close ports. When you want a quick and dirty scanning tool, there are some great free apps in the Market to do the trick. A quick search in the Market shows two apps which seem to be popular: OscanO and Port Scandroid.
This free app in the Market was written by Rich Jones of NewFreedomApps, found at http://www.thenewfreedom.net/ and the app is described by the author like this:
Port Scandroid is a port scanner for Android.