I use key words that appear in: request.ServerVariables("HTTP_USER_AGENT")

I convert the returned value to lower case and check for the following: (text, corresponding bot)
scooter,AltaVista
googlebot,Google
slurp,Inktomi
lycos_spider,Lycos
gulliver,Northern Light
infoseek sidewinder,Infoseek
architextspider,Excite
msnbot,MSN

I have not had this implemented long enough to determine if all of them work. I have seen MSN, AltaVista and Google run through though.