I have two macbook pros connected via firewire.
Both are also connected to wifi.
How can I check that traffic between the two computers is going preferentially via firewire, but everything else goes via wifi?
You will need to control this using network routing.
If you assign an IP address on a different subnet as the WiFi to both computers on the FireWire link - you will ensure that traffic between the two computers will be restricted to the wired network.
You can see the network routing tables by opening the Network Utility and you can control the IP address of both firewire interfaces using the normal network preference pane.