I'm not sure whether you can verify the firmware running on the device, but you can always flash it with the latest firmware from the website or your own build. That said, there is still bootloader that might be hacked, but that would have to happen in production because it cannot be overwritten. That would be equivalent to having a fake one.
Wait, I am confused. Do you mean that the firmware can be replaced after the device was assembled? If so, what is the procedure to do that?
Yes, it can be replaced. The official firmware releases are available from the trezor website and are signed by satoshilabs. you can load unsigned firmware and in this case bootloader will warn you ask you for confirmation that this is really what you want.