How to Mark Unread Comments?

I would like to mark unread comments – compare the last visit cookie with the date of comments and assign a class to ones with lastvistit < commentdate. Is there any plugin for this? I have found only plugins to show unread comments e.g. in sidebar, but this is not what i want, i just want to mark them as new.
P.S.: I have setup a cache on my server, so it probably cant by using php compare as it would show the same results for all visitors during each caching period.