When I come across a post that seems familiar or otherwise likely to be plagiarism I pick a distinctive phrase out of the post and do a google search like this:
"" site:bitcointalk.org
And if there is more than one result then I report the post and in the comment field I mention the number of hits returned by a google site search of the key phrase. This is less cumbersome than trying to put a link in the comment field to an identical post, especially when there might be *hundreds* of identical posts to choose from.
For example, the last post I reported had a peculiar phrase in it that I had seen before - "mining and swimming pool" - so I did a google site search of that phrase and it turned up 110 posts that were all identical (make that 113 posts as of now... busy little spammers!).
You can do that also. Whatever, way you go about it you'll come to the same conclusion. Even, if there's hundreds of links/sources before the post you only need to include one which happened before the post. You could even combine the two if you wanted.