if you scroll down below the quick reply box theres 'Currently Active Users Viewing This Thread' box when another member is using the 'Advanced Reply' function (not the quick reply) their username appears with a plus symbol i.e.
user viewing would look like this:- richydraper
whereas a user replying would look like this:- richydraper+
user viewing would look like this:- richydraper
whereas a user replying would look like this:- richydraper+