Pretty good, one would think. If nothing else, dogs know when we're happy, or sad, or angry (at them!). But what about gestures? If you point, does a dog know what you mean?
I don't know if dogs (versus chimps, or wolves, in the research that Patricia McConnell discusses in depth here (on 10/12/10) and here (on 11/2/10) inherently know what humans mean by pointing, or if they out of hand know that pointing is significant. There are studies using two cups, both smelling of treats, with a treat under only one of them. Chimps, apparently, fail to find the treat by following a human pointing at the correct one. So do wolves, unless hand-raised by humans. Dogs excel at it. Are they born knowing this? Maybe not. Can it be trained, though? Of course it can. We might train it without even intending to.
I'm speaking from experience, not science, or perhaps from what we'll call the "science of experience". As a for instance, I can give Elka the cue, "move", and point to the spot I would like her to move to, and she does. If we're walking about in the park, amongst trees, I can point in the direction she should proceed, if she looks back at me, and she'll go in that direction. I can point at an object and say "pick it up", and even if she's never picked that item up before, she will. Do I have a particularly in-tune dog? Do I just point a lot?
The Internet is a wonderful thing, which lets us experience many things that we would not otherwise. If you were to look up "canine agility" on YouTube, you could see many dog-handler pairs running agility courses with the handlers pointing at which piece of equipment to take next. If you look up sheep herding videos, the gestures there might be smaller (or larger, depending on proximity), but it seems there's a bit of pointing going on there. Whistling as well, which some dogs also intuit the meaning of with what seems like little conversion time.
Dogs are good at reading each other's body language when they're well-socialized (and even, sometimes, when they're not), and that kind of thing seems like it might be genetic behavior. Or instinct. If they're not the same thing. I recently read Calming Signals, by Turid Rugaas, which listed a whole slew of body language behaviors that dogs try to use to communicate with we humans, and with other dogs. I certainly am not, in the midst of my naval-gazing bloggery, going to be able to go back to the roots of communication and the relationship between people and dogs. But it's interesting to think about, certainly.