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 experience, other people would experience too; what you fear, desire, despise or admire, other people do. The main life situations are similar with all humans. Because you are of the same make, of the same species, because you live fit to the same world.
We all are the descendants of the same Sapiens ancestors "wired biologically" in the same way, resulting from the one evolution and the same Big History of life on our Earth.
The images and purposes flow in similar ways in our human heads, as we humans are brought up and function similarly, in comparable cultures.
Third, as long as we share a same language root, we also share the underlying ways of understanding and reasoning built into its structures of related words.
Fourth, as long as we share a similar language, environment and life-world, even more when we share having faced 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 similar and comparable, read the Essays of Montaigne  and chances are that you will see yourself in a mirror.
I add to this an even bolder claim, well known. The human mind has a power of empathy: to mirror, resonate, mimic, identify with, imagine and decipher everything human in a universally shared mental language of common sense. What is human made we are born to comprehend; whatever is felt, conceived and made by humans we are endowed to reconstruct and understand . Intuitively.
With this recipe you will not be far from truth, as long as you keep things simple. And your mind open and prudent, to check that your intuition is correct.
The differences - and there certainly are differences, often striking and dramatic and irreducible - come from the Cultures in which we were brought up, the personal histories, special circumstances, roles, identities, interests, obligations 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 experience and do if you were him, her, them, in their situation, by what you know what they may feel and know or ignore. Then look, listen and judge with common sense, as things keep changing while you think and act.
Do this and 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, need of certainties and territory, a place among the other to belong, to communicate, sex, pride, desire for power, hope and the need to believe, 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 care, listen, observe, 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.