There are ways that you show it indirectly. And when you don t express that anger directly to them, usually they resent it, and the frustration can cause people to withdraw from each other. Many people are not even aware they are angry, or that they re not expressing it. One way for people to tell whether they are angry is if they are short tempered. If you find your honking your horn at traffic, if you are not able to concentrate on your work like you want to, these are all ways of knowing that something is wrong.
Agitated feelings are good clues to unexpressed anger.
Also, there are occasionally some physical symptoms that go along with unexpressed anger, such as migraine headaches, peptic ulcers, upset stomach, tension headaches. Usually your body tells you that something is wrong. You are bottling something up, and you are not expressing those angers. You may in fact be in the company of others when you have these angry feelings, and you may want to find a nice quiet place where you can explain and express those feelings, or tell those feelings to the person you feel has caused them or at least is directly involved with you. So, it very much is a decision.