To look up a MAC Address on a remote computer you can use the build in windows tool called GETMAC.
Except from the MAC Address GETMAC can also supply information about network protocols and network adapter.
To get the MAC address of the remote computer PC123456 you simply type the follwing:
getmac /s PC123456
To get full details about adapter name and connection names aswell you simple add the parameter /v.
getmac /s \\PC123456 /v