You probably mean the "personal firewall" which comes with Tails, which protects the Tails system from other computers on the same LAN. This is not to be confused with a router or bridge firewall (or in large networks, a stand alone firewall computer) which protects the entire LAN.

If you have access to a second computer on your LAN, many live Linux distributions include nmap, so you can check for yourself. I've done that with earlier Tails editions and as I recall they did quite well.

If you have access to a second computer outside your LAN, you can check the LAN firewall. Some people like to use "Shields UP" from Gibson research. Note that if you surf there via Tor, you will wind up scanning the exit node in the Tor circuit, which is probably not what you want. (Although it shows there is much variation in how people configure servers which run Tor, possibly amongst other services.)

Ask if you want some tips for using nmap to check the personal firewalls of your own computers.