1. Does it have at least two women in it,
2. Who [at some point] talk to each other,
3. About something besides a man.
I bring this up as a point of interest, because of what it says about the blind spots of popular entertainment. Most Hollywood movies fail this test; if you extend #3 only slightly, to read "About something besides men or marriage or babies", you can strike out about 50% of the small proportion of mass-entertainment movies that do otherwise seem to pass the test.
The reason Bechdel's test is important is because it's a diagnostic indicator for the objectification of women. It's designed to identify the kind of film where, if two women talk to each other at all, the only subject of conversation is men (or babies). What it tells us is that our current movie and (to a lesser extent) our TV culture is pathologically misogynistic — be it in in the adoption of conservative Kinder, Kuche, Kirche values or the more extreme violence of women in refrigerators.
The current decade is characterized by security anxieties writ large, a socially conservative culture of retreat from liberalism, and a strong anti-feminist backlash. Our popular media, far from being the bastions of liberal values that conservatives say they are, are actually belwethers of popular culture, amplifying, reinforcing, and reflecting our culture's normative values back at us the silver screen. What they're showing this decade is really rather disturbing if you happen to agree with the core feminist ideological belief that women are real people too, not just baby factories and sex objects.
TV has always been bad — a hypothetical alien trying to make deductions about humanity by watching our TV signals would conclude that our normal gender ratio is four males to each female, and that's just for starters! — but of late, the messages coming at us out of the mass media are nothing short of toxic. If movies and TV objectified people of colour the way they do women, the only reasonable conclusion one could draw would be that a concerted propaganda campaign was under way to return us to the unquestioned institutional racism of the 1950s.
It's interesting to apply Bechdel's test to written fiction, although under some circumstances it breaks down; if the book you're analysing is a first-person narrative from a man's point of view, then it's relatively unlikely to pass: similarly if it's a depiction of skull-duggery in a mediaeval monastery (thank you, Umberto Eco). But it's a chastening warning when you apply it to your own fiction and find out that large chunks of it fail the test. I looked at my own novels: I've habitally made an effort to include strong female characters who are not just there to serve as a trophy or handmaiden for the Hero Protagonist, and even so, a couple of my books fail. Looking at my recent reading in the SF genre in general, the picture isn't good; while written SF comes off a lot better than Hollywood overall, with the exception of fiction set in all-male environments, passing the Bechdel test should be the norm, not an unusual occurrence.
PS: From now on I intend to start applying this test to my fiction before I embarrass myself in public. And (I realize this is offering up a huge hostage to future fortune) if anyone ever offers me a movie or TV deal, I am going to hold out for a clause in the contract requiring a scene lasting at least 30 seconds per hour of running time that passes Bechdel's test. Because? What hurts my fellow humans hurts me, and I can in conscience no more lend my implicit support to an anti-feminist backlash than I can lend my silence to a racist or homophobic campaign.