I think that a horror film is actually pretty obvious. If the film is designed to make you recoil in, well, horror then it's a horror.
It's also got to be the over-riding theme of the film. So Alien, for instance, is a sci-fi suspense movie with some horror elements. The horror element was actually secondary to the sci-fi story line and several of the deaths are hidden, making it more suspense than horror. It can be shoe-horned into horror as it contains horrific elements but it's not what the actual film is.
Saw is one that is less clear, but the premise of the film is to constantly make you feel under pressure of the building
horror of whether he will use the saw or not. It's a thriller too, but it's designed overall to horrify you.
It's certainly not cut and dry. Event Horizon is a horror in a sci-fi setting (it's almost a mirror of Alien), is Scream a thriller or a horror? Psycho? Boris Karloff's Frankenstein? Some times a film can change over a period of time, some times it can happily stride two or three (sub)genres. Scream is a thriller disguised as a horror film. It's actually a sub-genre "slasher flick" (I'm sure Jordan can correct me
) which are all that cross between horror, suspense and thriller. The horror is diluted to favour the suspense, but it's ever present.
In other words, most of the time it's a tough call, but "pure" horror is unmistakable (hence why the list is
sadly lacking in films like In the Mouth of Madness, Maniac Cop, Bad Taste, Phantasm II, Class of Nuke'em High, etc).