Chem Prof: Wears a white lab coat. This may actually be clean but does not have to be. P-chem profs have a brand new coat that has never been in the lab; polymer chem profs have strange glop on their coat, and intro chem profs have acid holes.
Physics Prof: Wears blue jeans and a flannel shirt. May sometimes forget to wear shirt altogether. If a professor is wearing blue jeans and suspenders, ten to one he is a physicist. Physics profs often have German accents, but this is not a distingushing characteristic. Be wary of psychologists with fake Viennese accents which can sound similar to the unwary.
Bio Prof: Sometimes wears a lab coat, though usually this is the sign of a biochemist. Marine biologists walk around in hip boots for no explainable reason, even in the middle of winter. They are apt to wear grey slacks and smell like fish, as opposed to most biologists, who smell strongly of formalin. Microbiology instructors go around in spotless white coats, refuse to drink beer on tap, and wipe all their silverware before using it. Never loan money to a bio prof, no matter how much he asks.
Psych Prof: Psychologists are not real scientists, and can be easily identified by their screams of protest whenever anyone questions whether psychology is a science. Psych people have beady little eyes and don't laugh at jokes about psychology. If you are not sure whether a person is a scientist or a comparative religion instructor, he is probably a psychologist.
CS Prof: Most CS profs are from India or Pakistan. You can tell by the gestures and accents. This is not a bad thing, though many of the American CS professors tend to pick up Indian accents which confounds more specific identification. Like mushrooms, CS students only come out at night, and, if not Indian, tend to take on a pasty appearance. CS professors do not use computers and therefore can be easily identified by their comparative good health with respect to their students. Many CS professors do not even know how to use computers, and are actually mathematicians or psychologists in disguise. Avoid these people.
Math Prof: Math profs are like physics professors except without any practical bent. A math professor will have only books and pencils in his office, as opposed to the piles of broken equipment that physicists keep. Mathematicians scorn the use of computers and calculators and often have difficulty splitting bills in restaurants. The easy way to identify a mathematician is by the common use of the phrases "It can be shown that..." and "Is left as an exercise to the student..."