If you look at the way the text lines up in terms of indentation, you can see which post it applies to.  Otherwise, I'd just cut and paste the text as a quote if you want to make sure people know that you are reply specifically to a certain post.

quote:

C hristian (no login) Jun 7, 2001 4:18 PM how can we tell whom one is responding to? whether one is responding to the original message, or to a previously posted response?