A simple idea - not needing any kind of extra coding... just mark the follow-up post in the title clearly visible with (Follow-Up) or (Follow) or whatever we decide on. See e.g. this post.

I know that it is not an ideal solution because you have to know about that convention but it's simple and there could be a hint in the FAQs...

Update: My original suggestion of (FU) could be easily misinterpreted in a way not intended :-)), thanks for pointing that out japhy.

-- Hofmator