We are used to work a lot with people, so we are very good in reading them, since most communication is non verbal. Great fun is to play the mentalist. The CrazyWaiter Knows Everything!  In fact you use some tricks.

The most important is cold reading. Without prior knowledge of a person, a practiced cold reader can still quickly obtain a great deal of information about the subject by analyzing the person's body language, age, clothing or fashion, hairstyle, gender, sexual orientation, religion, race or ethnicity, level of education, manner of speech, place of origin, etc. Also a knowledge of psychology (motives) and yourself are important for this. Who knows himself, knows the world!

Cold readers commonly employ high probability guesses about the subject, quickly picking up on signals from their subjects as to whether their guesses are in the right direction or not, and then emphasizing and reinforcing any chance connections the subjects acknowledge while quickly moving on from missed guesses.

Another technique widely used is shotgunning. This technique is named after a shotgun, as it fires a cluster of small projectiles in the hope that one or more of the shots will strike the target. Everybody has a relative who has diabetes.  All the Spanish girls are called "Anna", "Martha" or "Carmen" and if the mother says it's her, you say:"I knew there was a Carmen here"

When you hear people talking about something in another place (eg the buffet) you can refer to this. "I see that your grandmamma is called Annabella". Ofcourse you play with it by saying. "I see a lot of A's" etc. This is called hot reading.

Also you can make use of the The Forer effect. You say something that looks specific for the person, but in fact it is general enough to apply to a lot of people. It helps when you use mainly positively things like "You have a great need for other people to like and admire you. You have a tendency to be critical of yourself"

The last common technique is the rainbow ruse. This is a crafted statement which simultaneously awards the subject with a specific personality trait, as well as the opposite of that trait. With such a phrase, a cold reader can "cover all possibilities" and appear to have made an accurate deduction in the mind of the subject, despite the fact that a rainbow ruse statement is vague and contradictory. An example is "Most of the time you are positive and cheerful, but there has been a time in the past when you were very upset."

Have fun!!! I see a waiter that is going to predict some nice things about his guests in his next shift....

(part of the text is taken from the linked Wikpedia pages, under CC. Image published under PD. And here's a nice WikiHow about Coldreading)