You may want to check out a free program: BotSplit at http://www.BotSplit.com (disclosure: I wrote the program).
This programs operates offline to do an aggressive job of rooting out robot visitors. Note that there are 7 rules used and listed. Checking for access to robots.txt is a subset of one of the rules. Some of rules are applied after the current session restarts and others require visibility of the entire session.
Thus, trying to do this in real time before access is made is *tough*.
As to what you are trying to do: tell visitors which visitor number they are, let me suggest calculate the number of human visitors offline and reset your online count periodically. Then increment your online count either with every visitor or a fraction representing experiience.
Note that tracking IP's of Bots is an exercise in futility. The MSDN robot used 37 different IP addresses in one run alone.
