This is the secret key to knowing people as they are, what they feel, how they may judge, what they may do:
First, what you feel, other people would feel; what you fear, desire, despise or admire, other people do.
Second, the images and words flow in similar ways in our human heads, as we humans are built and functioning similarly. We all are the descendants of the same Sapiens ancestors "wired biologically" in the same way, resulting from the same evolution and the same Big History on our Earth.
Third, as long as we share a same language we also share the underlying ways of thinking built into it.
Fourth, as long as we share a similar environment and life-world, even more when we face the same situations, needs, crises, challenges, questions we also share the choices of responding to them. (Even as we may chose quite differently).
If you do not believe my hint that we humans are all the same, me read the Essays of Montaigne  and chances are that you will see yourself in a mirror.
I add to this an even bolder claim: the human mind has a power to decipher everything human in an universally shared mental language of common sense; whatever is conceived and made by humans we are endowed to understand .
With this recipe you will not be far from truth, as long as you keep things simple.
The differences - and there certainly are differences, often striking ans dramatic and irreducible - come from the Cultures in which we were brought up, the personal histories, special circumstances, roles, identities, interests and points of view. These you can also find out or recognise if you care to inquire and observe.
Start with asking yourself what you would do if you were him, her, them, in their situation, then look, listen and judge with common sense, as things keep changing while you think. You will become a fine psychologist. Forget the complicated theories.
Basic things push people, or pull them to do what they do. First things act first: fear of death, urge to be safe of pain, fear and loss, need to own things, certainties and territory, a place among the other to belong, to communicate, sex, pride, desire for power, hope, being who you truly are, ever better  and later if not last, being civilized and good, spirit divine - other than beasts. Very rare are the perverts with an urge of death and a love for evil for evil's sake. Humans want forever more, at least for their children. One need satisfied, another arises. You will never have everything you need or wish. Nobody will. Count on this without reducing persons to such simplicity; your good sense will quickly enrich the basic model with the facts and nuances you know or observe. Essential strokes will enrich the portrait when you enter dialogue and do things together with people.
 Essays of Michel de Montaigne, Tr. Charles Cotton, 1877
 Reading Giambattista Vico encourages me in this. Vico, Giambattista, First New Science, The  (Cambridge, 2002)
 Maslow Abraham, Motivation and Personality, Harper & Row, 1954
 Maslow Abraham, A Theory of Human Motivation, Psychological Review, 1943, Vol. 50 #4, pp. 370–396.