Sites like Wikipedia and Google (among others) don't let you sign up through a proxy or Tor (at least with Google, you have to provide SMS verification if you sign up through proxy.) Somehow they detect it that you are behind a proxy.
Is there a workaround for that? I'm thinking of a service which tell these site you are not behind a proxy, even though you are. Is there such a thing?
There is no magic way to detect a proxy. What they are likely doing is looking for IPs that have lots of different users coming from them or that are published as Proxy end points. A proxy that has a limited number of clients and doesn't publish itself as such would seem no different from any other internet user. The problem is that such a proxy would need a large number of IPs to avoid being detected as a proxy and would thus be difficult to manage.
Some proxies send the X-Forwarded-For header, which can be used for proxy detection - in addition to the checks for known IPs and excessive/unusual traffic.
To build a service like you are talking about not only would you have to distribute traffic across a large number of IPs to avoid detection as AJ indicated, but also ensure that you are sending (or forwarding) headers that leak client information.
Tor also maintains a directory of exit nodes, which is public. So, it's easy to detect that a user is coming in via a Tor proxy by simply checking if their origin IP is listed as a Tor exit node.
