Some would say it's being perceptive. I would add being systematic, analytical, curious, questioning and sceptical but not cynical. At the risk of incompleteness there are other aspects I won't go into here.
Gene at Harry's Place has an account of a piece by Sue Blackwell that mentions how anti-Zionism can just be an outpouring of anti-Semitism. In it she mentions how
[The Protocols of the Elders of Zion] crops up everywhere on the internet, including the weblogs of people who claim to be campaigners for Palestinian rights. I had a graduate student in my office not long ago, a highly intelligent young man who is a member of a socialist party in the UK. He told me in all seriousness that I really ought to read this incredible exposé of a world Jewish conspiracy, which was apparently new to him.How much reading of the history of socialism and general socialist material do you have to read before you come across condemnations of the viciously anti-Semitic Protocols? Sue Blackwell's "highly intelligent young man" should read more books and he should read better books.
David Hirsh has written a splendid reply to Blackwell's piece. Here's a snippet:
In this article, Sue is recognizing the fact that her brand of ‘antiracist’ anti-Zionism lives in a world where there are also a number of antisemitic anti-Zionisms. And she is confirming the fact that the boundaries between these different but adjacent movements are sometimes fuzzy and are sometimes porous. Political alliances across those boundaries are looking increasingly tempting to many anti-Zionists.Go read the whole piece.
The question that follows from Sue’s new piece is what kind of political responsibility she is now prepared to accept for these worrying developments. What does the fact that some of her own rhetoric is very similar to some of the rhetoric of the antisemites tell her? How does she explain the fact that antisemites assume that she is one of them? How does she explain the fact that a highly intelligent postgrad student, a member of an anti-Zionist socialist party, knocks on her door all over-excited at having discovered in the Protocols evidence for a global Jewish conspiracy? Why are anti-Zionists, who go out of their way to intervene in Israeli and Palestinian affairs, so often unwilling to educate themselves and their recruits in the history and themes of antisemitism?