There is a website with a test that tells you if a particular site on the internet is blocked in Iran, predictably called Blockediniran.
I can't find any source that can give information about the reliability of this test, which I presume will not be 100%. In fact, while the OK answers (not blocked) are definite, the BLOCKED results are more cautious, saying: "It appears as though this site IS blocked in Iran".
Among the websites banned in Iran are Jihad Watch, Daniel Pipes, Fox News and The Daily Mail.
Very understandably not blocked are The Guardian, The Times, Occupy Wall Street, Noam Chomsky, Naomi Klein.
My site, Enza Ferreri, is blocked in Iran.