Have you ever questioned people's intentions? If yes, what are your first signs of those?
Yes and No. First, there are those innocent ones who don't even know their intentions, like my INFP friend who says X but actually meant Y. In such cases, you have to foresee the change of course and intercept them at predicted coordinates.
And as for dishonest intentions: Your question could be rephrased as "How do you know someone's lying?". It's in the way people talk, behave, and also in their eyes. In the way people talk, it's that subtle bit of exaggeration (mostly Se types) that is crafted along the lines of clichés with the intent to convince. Se types usually pride themselves of being original in this regard.
And as for true Machiavellians, it's intuition about the subject that is not in accordance with the person's description of that subject. This is actually more reliable (and the only thing you actually need to know) than taking the direct route by reading the person. And then it's that "waiting for the other person's move" that was just a bit too long.
So much for the things that are describable.