This addon wasn't created by me, I just share this!
http://wow.curse.com/downloads/wow-addons/details/arenaspy.aspx]ArenaSpy
What does ArenaSpy do?
ArenaSpy will automatically detect your enemies during warmup if they are from the same server and faction.
How do I use it?
It will automtically announce enemies to your party by default, but you can change this by typing "/as announce type", where type can be "self", "party", "raid", or "raidwarning".