There have been many studies on this subject. One interesting one showed that dogs were able to tell when their owner was thinking of going for a walk even when the owner was in a different room. It has also been shown that some dogs know when their owners are nearing home even when the time of day is different to the normal time for them to be expected. There is a sense that dogs have and we don't, although some people have a degree of it too!
Dogs are also constantly aware of things that are too subtle for us to notice, sweat production, heart rate and breathing rate for instance, that help them assess the emotional state of their owners.