You can use the Windows getmac command to get a MAC address from any Windows box if you have the hostname or IP.
C:\>getmac /s 11.11.11.50
Physical Address Transport Name
=================== ==========================================================
00-13-8B-3E-43-D0 \Device\Tcpip_{EE7D131B-C172-411B-7F12-E81F82057FEC}
N/A Hardware not present
C:\>
Router1#sh arp | i 43d0
Internet 11.11.11.50 0 0013.8b3e.43d0 ARPA Vlan777
If you are on the same IP subnet as a device use the arp command from a windows device.
C:\>ping 11.11.11.50
C:\>arp -a
Interface: 10.11.11.116 — 0xb
Internet Address Physical Address Type
11.11.11.50 00-13-8B-3E-43-D0 dynamic