It might be a coincidence, but I'm sure my younger cat Minnie can tell when I'm sad, as she always comes over and gives a questioning little "Meow?". I definitely think animals can detect if you're nervous or anxious though, especially. A lot of the time they see you as the "pack leader" or "alpha animal", and they're looking to you for guidance! So if something concerns you, then it should definitely concern them too. Of course, try telling this to a cat climbing a 20 foot tree. Then it's all "I'm fine, mum! Don't worry!".
I think dogs would probably be even more perceptive of human emotions than cats, as well, because their emotions seem to vary more in general. If you yell at a dog it will become sad, but if you yell at a cat it'll just get scared by the loud noise (mostly. Of course I'd never yell at my kitties anyway!).
