Just did a quick web search on ' how to discern people's motives' and there's quite a lot of material - others who share the same problem may wanna have a glance at some of that stuff. Just looking thru some now.
People are motivated by themselves and their belongings. I don't believe there's something that really helps in discerning whether or not the person is hiding something or is being really kind and friendly. All you can do is to check it over time.
I enjoy hearing and asking people about what they are thinking about other people. What they think about others, how they act about them, how they react in this or that situation, is likely to be how they'll behave with me. 100% sure that I won't be the exception for this person. That's how I get my informations. Most people don't lie when they tell you about their stories or the stories of other people or talk about their morals and so on. I've met people who lie about this through, the best way is ALWAYS to rely on the actual actions and facts.