How can I check if the motherboard has been replaced?
The MAC address is linked to the logic board. So, if the logic board has been changed, MAC address will have changed too.
I've been told this by a Genius et Genius Bar when he said they need to change my logic board and asked if my router was checking MAC address for allowing net access, as I will then have to reconfigure it.
It also seems to be backed-up by this technical note (look for first occurrence of "logic board").

Edit: For your previous MAC addresses (ethernet and wifi and bluetooth, as I suppose they all change on a logic board exchange), you will find them on your computer's box, if you still have it (but it seems to be the case:)). These are the Ethernet ID, Airport ID and Bluetooh ID. You can then check in System Profiler to get the current ones.