What IS good, and how do we determine it?

[Wyrd of Gawd]

This is kind of a spinoff of the WHY BE GOOD thread.

The question I have for atheists, isn't "why by good." I think it's simplistic and deeply flawed to think that the only reason to "be good" is to avoid Hell. And of course, I believe that anyone can be a good person regardless of beliefs.

The question I have for atheists is how do we know what IS good?

Religious or not, we all somehow know that certain things are intrinsically, universally immoral. Let's use murder as an obvious example. So if murder is wrong, where did this law come from? If this is a universal truth, where did this truth come from and who/what determined it to be what it is?

If an act causes you pain, suffering, and loss chances are you will consider it to be "bad".  Why do you get upset when someone smacks you upside your head?  If it's not "bad" why get upset?  Do you get angry when someone takes your stuff?  Why?  You got it before so chances are you can replace what the person took from you.  You will then have new stuff and he will have your old worn out stuff.  Isn't that a good deal?