Somehow packet sniffers can do that -- they catch HTTP requests (POST, GET etc.) sent from the local computer. For example, this is a search form request sent from FoxPro Wikis Page and recorded by HTTPLook 1.1:

POST /wc.dll?Wiki~DisplayFindResults HTTP/1.1
Accept: image/gif, image/x-xbitmap, image/jpeg, image/pjpeg, application/vnd.ms-excel, application/msword, */*
Referer: http://fox.wikis.com/wc.dll?Wiki~DisplayFindResults
Accept-Language: en-ca
Content-Type: application/x-www-form-urlencoded
Accept-Encoding: gzip, deflate
User-Agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; .NET CLR 1.0.3705)
Host: fox.wikis.com
Content-Length: 16
Connection: Keep-Alive
Cache-Control: no-cache
Cookie: Wiki=username=AnatoliyMogylevets; WikiSessionId=1000HI3RV

Allsearch=SYS(15)

Another good tool -- even more powerful -- EtherDetect. And many similar products can be found online.