I am not a hardcore Twitter user but this sounds like a stupid question. In Twitter there seem to be no way to track where conversations start. I can only see that @replyto, leading me to the user account.
But he has a lot of tweets, I as a user will not know where the conversation started?
Best Answer
There's nothing built into Twitter itself.
There's a service call Bettween that offers this. It's a little slow the first time you use it, but claims it's quicker for subsequent visits.
Additionally this post on Gamblz from 2008 lists 5 ways to find and track conversations using 3rd party applications. I don't know whether any of these are still working.