I grew up riding arabs and now I have one of my own. Arabs are a very distinct breed. You can tell by their high tail carriage and their slender frames. Like CrazyChester said, they do have one less vertabrae in their back although not all arabians have the dished faces. That's common for egyptian bred arabs, but not for other strains like the polish bred horses (see my horse for a reference to a polish bred arab). Their heads are smaller than most other horses though. They also have a habit of arching their necks and a lot of the time they have longer necks.
Arabs are very strong. They were bred to run long distances in desert climates. They're a kind breed, but very smart and opinionated (arabs get bad rap from a lot of the horse world for this) so you have to be careful to getting training in early and patience is a must. Otherwise, its easy to have an arab get out of control. They want their rider to be their partner, not just someone telling them what to do. I love Rocky bc he is extremely smart and can figure a lot of things out for himself. Arabs are also very sensitive horses so any mistreatment can result in deep trust issues and behavioral problems. They're excellent edurance horses as well! I hope this helps. Every horse is different and a few may not conform to what i've said, but these are the traits i've observed from working with them for so many years.