The first thing I notice is the facial expressions and eyes. It shows you a tell tale sign whether they are happy to see you or not.
Speaking to someone may not tell you the truth or their feelings but you learn a lot from how they act. I always look someone in the eye and if the person is genuine they wont look away. The ones that look away normally are lying and not being fully honest with you. This sets my guard up straight away and has helped me read people.
The more information that you can read into someone the better you are equipped for dealing with whatever you need to overcome. When i meet a customer for the first time I can tell by his/her reactions if they are serious not and how much work I need to put in to making them change their mind.