How to tell if web visitor is a robot

[LeeCee]

On an ASP website, is there a way to tell whether a visitor is a robot?I'm thinking there might be a parameter in the \[code\]ServerVariables\[/code\] collection that could be used, in a similar way to \[code\]HTTP_X_FORWARDED_FOR\[/code\] and \[code\]REMOTE_ADDR\[/code\] can be used to get the visitor's IP addresses.Searches on Google have so far yieled few leads.Thanks for your help.