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.