evolving to the tdd mindset

while reading through Beginning Ruby on Rails E-Commerce, my eyes have been really opened up to test driven development and how important it is.

i have a problem though. i don't think like a test driven developer... i see these tests, and while they're very insightful on how to write test code, what i'm really looking for are testing guidelines. what should i test? what should i just assume works, because the code i've used 100 times before has always worked?

while going through the book, i'm thinking thins like "i wouldn't have that to test and see if the h1 tag was on the page". or then there are things that i wouldn't even know _how_ to test because examples weren't given in the book :wink:

i realize that it's a different mindset, and i also realize that it won't happen overnight. but what are things that other developers use as a rule of thumb in order to decide how their tests should run? what questions can i ask myself as i'm writing each action that can help me make decisions or at least help train my mind to start thinking about my tests.