Well, the only real ways you could tell you have been exposed to a communicable disease is to either become ill from said disease; ie, Conjunctivitis or "Pink Eye" becoming prevalent when infected fecal matter is introduced to the eye. The other way is if say you were working with someone who is irregularly coughing or sneezing, which could indicate diseases, that is to say if someone has signs of a disease with sneezing, coughing, diarrhea, or any other way of transmitting it is communicable. If you work in healthcare though, you'd most likely either have a patient you'd work with tested, or yourself be tested for possible exposure; TB Skin Tests actually are positive if you are "exposed" (you don't have to be infected).
Potential ports of entry for an illness are usually the places that are the orifices or "holes" to the outside world. The easiest are of course needle sticks, which could also involve breaking of the skin, in which case you'd have a entryway. Additionally your mouth, nose, ears, eyes, the pores of your skin (most of the time are so small and protected nothing can get in, except drugs), rectum, urethra, and (in females) vaginal opening.
