Quite simple but tricky. You need to watch them how they behave in your presence and absence. Sometimes, the time lag between these two events might be long. So, your memory to recollect, and skill to link and understand the old and new aspects are vital. But any misinterpretation on your side will damage your value.
Honest people tend to behave same way in all places at all times. They will not twist and contradict their own statements unless due to memory problem.
People having chameleon attitude will act and speak depending upon the situations they are in. They pretend to be good but act differently. Because, they are selfish and concerned for their purpose even at the cost of self-dignity. However, such people do not exhibit this trick in the presence of certain persons whom they perceive as Intelligent to detect their mischief, and which might go counter-productive.
One classic example is our politicians. They will make tall promises to us and criticise the opponent party during polls, but switch sides and seek favours from the same party which they derided earlier.
So, it is a matter of Morality that decides any individual’s behaviour.