When the Ukip politician Godfrey Bloom referred to "Bongo Bongo land", there were not many who denied the remark was racist. When the same man told women who failed to clean behind the fridge that they were "sluts", most could see the comment was sexist. Yet when the target of an insult is a Jew or Jews, there is rarely such certainty. Unless antisemitism comes dressed in an SS uniform and doing a Hitler salute, we are regularly thrown into confusion. Suddenly we are in the seminar room, calling on experts to tell us whether or not this or that sentence was anti-Jewish, the debate usually ending without clear resolution. To add to the complexity, very often Jews disagree among themselves, with just as many willing to give the disputed word or deed a free pass as to condemn it.

So it has been this week with the Daily Mail's sustained assault on the late Ralph Miliband, the Marxist scholar it branded "The Man Who Hated Britain". Some detect a whiff of anti-Jewish prejudice, some swear there is no such thing. When pressed on the point by the BBC, Ed Miliband himself declined to add antisemitism to his list of charges against the paper.

All of which, I imagine, must make it hard for the open-minded outsider, the non-Jew keen to oppose all forms of racism. They know they're against antisemitism, but how exactly to spot it? When is the line crossed? Where, in fact, is the line? In the spirit of public service, let me attempt an answer.

First, the word itself. So much as mention antisemitism and someone will pop up to tell you that Arabs are semites too so why do Jews insist on hogging, as it were, all the antisemitism for themselves. But the word was not a Jewish invention. It was popularised by a 19th-century German Jew-hater called Wilhelm Marr, keen to put his loathing on a pseudo-scientific basis: he used "semites" to mean Jews and, partly because "anti-Jewish racism" is a mouthful, the word has stuck.

Despite the name, it is not a phenomenon safely buried in the past. Just because hatred of Jews reached a murderous climax in the 1940s does not mean it ended with the war in 1945. It is alive and well even in 2013. Whether it's on Twitter or in the cartoons that routinely appear in much of today's Middle Eastern press, crude slurs and hideous caricatures of Jews – hook-nosed and money-grabbing – endure.

Move away from the gutter, however, and antisemitism is rarely so obvious. It is communicated through nods and winks, hinted at rather than spoken. In Britain especially, prejudice against Jews has long been of the latent, rather than overt, variety. Even the words Jew or Jewish are often avoided: spotting the euphemisms – "flamboyant North London businessman" – is a pastime in its own right. So those ready to acquit the Mail because there was no bald, outright statement of antisemitism were probably using the wrong measure.

Instead, there are familiar tunes, some centuries old, which are played again and again. An especially hoary trope is the notion of divided allegiances or plain disloyalty, as if, whatever their outward pretence, Jews really serve another master besides their country. Under Stalin, Jews, especially Jewish intellectuals, were condemned as "rootless cosmopolitans" (another euphemism) lacking in sufficient patriotism. The Mail's insistence that Miliband Sr was not only disloyal but actively hated his country fits comfortably in that tradition.

In the antisemitic imagination, Jews are constantly working for some other, hidden goal. In this, antisemitism stands apart from other racisms, which tend to view the hated as straightforwardly inferior. Antisemitism is instead a conspiracy theory of power, believing that the Jews – always operating as a collective – are bent on some grand plan of world domination. Which is why images of Jews as puppet masters, or of having the world in their "financial grips", as Baroness Jenny Tonge so memorably put it, always hit a nerve.

In the last century, antisemites of left and right diverged on exactly how the Jews planned to enslave the human race. Jew-haters of the left believed capitalism was the preferred method, while antisemites of the right reckoned communism was the Jews' chosen tool, with Marx and Trotsky the fathers of an imagined "Judeo-Bolshevism". The Mail's dogged exhumation of Miliband's Marxism, buttressed by references to Eric Hobsbawm and Harold Laski – funny, that of all Miliband's many colleagues and comrades, the paper highlighted two who happened to be Jewish – carries a potent echo of that unhappy history.

And always on hand for the antisemite is some reference to Jews' religious practice, real or imagined. For centuries, those who hate Jews would throw the phrase "chosen people" back in their faces, falsely interpreting it as a mandate for Jewish supremacism. Others would claim that Jews feasted on the blood of Christian children as part of their Passover ritual, the lethal "blood libel" that prompted anti-Jewish pogroms and cost Jewish lives for centuries. It might be a reminder that Jews were still chained to the Old Testament alone, unenlightened by the gentler, more forgiving teachings of Jesus. This is why I and many others – previously ready to give the Mail the benefit of the doubt on the matter of anti-Jewish prejudice in its coverage of Ralph Miliband – stopped at the reference in Tuesday's editorial to "the jealous God of Deuteronomy." That looked like another veiled pointer to both Miliband Sr's indelible alienness – and his membership of an ancient, vengeful people.

Ah, but hasn't the Mail been defended on TV by Jewish employees and hasn't the odd rabbi said they did nothing wrong? This happens every time, too. But it's not much of a defence. Every feminist knows a woman can always be found to say she sees no sexism, no matter how grave the offence to her fellow women. Why is it a surprise that some Jews will decide it doesn't suit them to make a fuss, that they'd rather keep their heads down and get on? After all, it's only antisemites who believe Jews operate not as individuals but in lockstep with each other, in pursuit of imagined Jewish goals. For that reason, it is neither here nor there that a Jewish reporter wrote the original piece. Besides, the most toxic elements were the headline and subsequent editorial – and those are the responsibility of the editor.

Antisemitism can seem a subtle, elusive business. Calling it out can feel too much like hard work, often prompting a torrent of abuse as hurtful as the original offence. But it has to be named for what it is – and not only by Jewish writers like me. History could not be clearer on this last point. Antisemitism may start with the Jews – but it rarely ends with the Jews.

Twitter: @Freedland