in another word, how do you know that you have tested enough?

new to testing, I just started writing test for the webapp i wrote, using Test::More and Test::WWW::Mechanzie (i wrote the webapp first. otherwise would be interesting to try test-first before writting code). with corner cases and bad inputs, i feel that i have tested the webapp enough ( so it works the way it should ) along with T::W::M testing, i also test module requirements, configuration file. so far it has been joyful.

however, based on what i read from here and perldoc, i seem to missing unit test. i.e test each function and module API. I think Test::Tutorial has an example on this. i also read this: 

Bill Venners: When do you stop writing tests? You say in Refactoring, 
+"There's a point of diminishing returns with testing, and there's a d
+anger that by writing too many tests you become discouraged and end u
+p not writing any. You should concentrate on where the risk is." How 
+do you know where the risk is? 

Martin Fowler: Ask yourself which bits of the program would you be sca
+red to change? One test I've come up with since the Refactoring book 
+is asking if there is any line of code that you could comment out and
+ the tests wouldn't fail? If so, you are either missing a test or you
+'ve got an unnecessary line of code. Similarly, take any Boolean expr
+ession. Could you just reverse it? What test would fail? If there's n
+ot a test failing, then, you've obviously got some more tests to writ
+e or some code to remove.

[download]
from http://www.artima.com/intv/testdriven4.html

It reads fine. but i got the feeling that i am going to write hella more tests if using his method. now for the experienced monks, I am interested in how much unit testing or just general testing do you do?

In reply to when do you stop writing test? by Qiang

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