What to Unit Test

Testing of a class should verify that: 1. methods and properties return expected values
2. Appropriate excepts are thrown when an invalid argument is supplied
3. Interactions between the class and other objects occur as expected when a given method is called

Crippledsmurf - http://stackoverflow.com/questions/62625/how-do-you-know-what-to-test-when-writing-unit-tests

Should you test internals (e.g. setting of private variables)? Some say no. On the other hand:

While it is true that there is a certain brittleness where test code knows internal details - this is unit testing, which is, by nature, whitebox, not black box.

http://googletesting.blogspot.com/2009/02/to-assert-or-not-to-assert.html - comment by Christian Gruber