How do I know if I'm good? If I'm a fresh grad I have no idea if the salary I'm getting is good. Without an objective measure of skill and without knowing how "good" other people are its pretty hard for a greenhorn to know.
That's why a greenhorn asks everyone how much they earn. Screw politeness and anglosaxon pride and whatnot. Ask everyone. Tell everyone. Observe reactions.
When you get to the point where nobody comments on how cheap you are. That's when you've reached the bottom limit of what you should be earning.
Ask for more.
Also, if you start out as a freelancer, you can iterate your paycheck every couple of months. Do that. Ask each new client for 25%+ more than your previous client. Eventually you will find a ceiling.
Figure out how to go beyond the ceiling.
Encourage all your friends to do the same. This gives you an environment that always pushes you to earn more.
That's what happens with a market starved for good people.