In many countries, it is no secret that government censorship of the Internet exists, as documented in the book Access Denied: The Practice and Policy of Global Internet Filtering, edited by Ronald Delbert, John Palfrey, Rafal Rohozinski, and Jonathan Zittrain (http://opennet.net/accessdenied). When a popular site is widely blocked, that fact tends to become widely known within the country.
But, in general, determining whether someone is preventing you from accessing a Web site or from sending information to others can be difficult. When you try to access a blocked site, you may see a conventional error message or nothing at all... the behavior may look like the site is inaccessible for technical reasons.
Some organizations, most notably the OpenNet Initiative (http://opennet.net), are using software to test Internet access in various countries and to understand how access may be compromised by different parties. In some cases, this is a difficult or even dangerous task, depending on the authorities concerned.
In some countries, there is no doubt about government blocking of parts of the Internet. In Saudi Arabia, attempting to access pornography results in a message from the government explaining that the site is blocked, and why. In countries that block without notification, one of the commonest signs of censorship is that a large number of sites with related content are inaccessible for long periods of time, except perhaps when they take countermeasures such as moving to a new domain. Another is that search engines return useless results or nothing at all about certain topics. These may be related to pornography, gambling, drugs (including alcohol) or other illegal activities or to political or religious movements deemed dangerous (for example, neo-Nazi sites blocked in Germany).
