If instead someone "helpfully" redirects me to something else, then I can't reliably detect the condition. (My home ISP does that to me, and it gripes me to no end.)
Most times I've seen this malicious behaviour, it was "just" a DNS manipulation, i.e. it happend only for domain names not currently found in the DNS. Instead of returning NXDOMAIN, the provider's DNS server happily returned the IP address of a provider-supplied web server that offered some unhelpful search form. Verisign started this malicious behaviour in September 2003, and several providers happily copied it. Bypassing this malicious behaviour is quite easy, just don't use the DNS server(s) of your provider, but some free ones. Google offers 220.127.116.11 and 18.104.22.168, I use them in my DSL router (that acts as name server for my LAN), because my provider "accidentally forgets" my opt-out every few weeks, and I'm quite happy with that setup.
If you see the unhelpful page instead of a 404 page also for URLs with well-known servers, like http://www.google.com/google/does/not/have/this/page, there are essentially two methods left: Either your provider infected your PC with that malware when you installed software for access to your providers dial-up network, or your provider uses a transparent proxy. Booting from a live linux cdrom easily shows what really happens. In the first case, the unhelpful page is gone as soon as you don't use the malware-infected operating system. The second case can be identified by the fact that any web server you connect to sees your request coming from a different address than the one your provider issued to your system. Pages like http://ix.de/ip show you from what IP address your request comes from.
Today I will gladly share my knowledge and experience, for there are no sweeter words than "I told you so". ;-)
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