I did it like this:
Assign ip-addresses to the cards...
Just connect one card. See if you can ping the computer on the expected ip. (Also try the other ip, I experienced that it will listen to both ips on both devices for some reason).
If it only responds to one ip, you also figured out which device you're currently plugged into
Otherwise use ifconfig ethx down to shut one device down manually. Can still ping? You were on the other device. cannot ping? Well, also found out which device is which card...
For testing you might also want to do ifconfig ethx up and ifconfig ethy down to see if the behaviour inverts.
Have fun, Steave.