Can you explain a little better? Are you asking one of these two things?
How can you determine if Google or a certain web2.0 property has cached the comments on one of your blog posts?
or are you asking
How can google or certain web2.0 properties determine if the comments on your blog are "fresh" or "cached" (or maybe "syndicated" from youtube/amazon/yahoo answers)?
I know, probably neither. Sorry, just not getting it.