There are two ways.
Physically inspect the card. the MAC address is usually printed on the card.
Run "ipconfig /all" and observe the output:
Ethernet adapter Local Area Connection:
Connection-specific DNS Suffix . :
Description . . . . . . . . . . . : 3Com 3C920 Integrated Fast Ethernet Controller (3C905C-TX Compatible)
Physical Address. . . . . . . . . : 00-06-5B-26-F8-6C
Dhcp Enabled. . . . . . . . . . . : No
IP Address. . . . . . . . . . . . : 10.200.1.50
Subnet Mask . . . . . . . . . . . : 255.255.0.0
Default Gateway . . . . . . . . . : 10.200.1.1
This is a snippet of the configuration of my desktop PC at work. the MAC address is the "Physical Address". Note that some network cards allow you to change this.