Some ways in which you can rely on to know whether you really know a person includes knowing about the person's family. This helps you know why a particular person may act or believe on certain ways, especially if the whole family acts in the same manner. You can also check on how the person acts in relationships in the past and whether he/she has good relationship skills.
When they constantly act the same way, and you know you can count on them. This might take months, but you will soon get a good feel for how well you know a person, just make sure you stay the same kind of person, otherwise they might also react differently to you. Just act always be supportive and positive, don't judge others, and try to see the mistakes other people make, and see it in your heart to forgive them.