From The Telegraph: If you are worried that your dog is not paying attention to what you are saying simply look at where is he is staring.
Dogs look to the right when they recognise familiar commands because they process the sound in a specific part of their brains which remembers that the sound is important. Researchers at the University of Sussex discovered that dogs respond to speech in much the same way as humans do, processing recognisable sounds in the left hemisphere of the brain. In contrast, unusual noises or speech,are processed in the right hemisphere of the brain. However due to cross-wiring in the brain, if a dog turns to the right it means the left side of the brain is processing the words and vice versa. Researchers looked at how dogs responded when their owners told them to ‘come on.’ When presented with familiar spoken commands, in a usual tone, dogs showed a left-hemisphere processing bias and turned to the right, indicating that they recognised what was being said. However when the command was said in a different tone and speed, the dogs could no longer process it as a familiar sound and so looked to the left.