With addons you can do it two ways that I've seen so far.
If you're marking your mob with a mark (skull, square, triangle), you can show on your raidframes if you're using Grid or Vuhdo, what your raid members are targetting in an indicator showing this mark. That way you scan your raid and watch for raid marks (that in this case will show their target's mark, not their own mark) which should tell you who's doing what.
Otherwise, use an addon that will list in a tooltip who's targetting a mob. Like:
http://www.wowace.com/addons/magic-targets/
http://www.wowinterface.com/download...93-TipTac.html
Some unitframes may also offer you the option to list all raiders targetting your currently targetted unit, like XPerl.