First we install nmap from the yum repository, if your using Cent OS | RHEL | Fedora, in may case I am using Cent OS 6.
yum install nmap.i686
After nmap has been installed, use the command below to find out the mac address mapped to the IP.
# nmap -sP 192.168.1.98
Starting Nmap 6.25 ( http://nmap.org ) at 2013-04-01 09:50 PHT
Nmap scan report for desktop12 (192.168.1.98)
Host is up (0.00016s latency).
MAC Address: C8:60:00:D0:69:5F (Asustek Computer)
Nmap done: 1 IP address (1 host up) scanned in 0.04 seconds
If you have no idea what the client IP is you can also use nbtscan if the client are Windows machine to find out any useful information.
# nbtscan -v -s : 192.168.1.0/24
192.168.1.0 Sendto failed: Permission denied
192.168.1.98: __MSBROWSE__ �:01G
*** stack smashing detected ***: nbtscan terminated
If nbtscan is not installed, just do a search on your yum repository and install the package.