Network devices parameters are stored in registry:

HKEY_LOCAL_MACHINE/SYSTEM/CurrentControlSet/Control/Class/{4D36E972-E325-11CE-BFC1-08002bE10318}

 Each device has it's own registry folder : 0000, 0001 , 0002 etc. , inside AdapterModel field will inform you specifically with which device you are dealing.

 Now other there are two possible options:

1) Device is using it's fabric MAC address , the NetworkAddress field will not appear

2) Device is using a spoofed MAC address, the NetworkAddress will contain it's value

To determine real hardware MAC address:

Find the device in registry, check if NetworkAddress field appers

(It's missing): Just get the MAC with any method (WMI, GetAdapterInfo  or NetBIOS) and you are set.

(It's there): 

1. Backup the value
2. Remove the NetworkAdress field from device's registry key
3. Restart network device(that's the most evil part...)
4. Get the real hardware MAC with any method(see above)
5. Restore the backup value in the registry(recreate the NetworkAdress field)
6. Restart network device(evil part strikes again)

Voila!