The small piece of knowledge that you are missing is that Teachers (or indeed Administrators) do not see activities in the same way as Students. For example a Teacher never gets a Grade when test a Quiz.
"Do I have to make a dummy account and enrol to test everything? I can see resources else I've added to a course -- labels, pages, etc. Can I not preview any activities?"
That is in fcat absolute best practice
Although you may be able to "switch roles" (if you site is setup to allow this) it still isn't 100% the same as what a Student will see.
Resources will look the same to everyone, because you don't "do" anything with them - just look at them.
So yes, create a test Student (dummy user), and enrol that user in every Course (site wide) - so you know you can always quickly check what a Student sees in any course instantly !
And an even better tip, just open a second browser (Firefox, Chrome, Safari, whatever) and Login as the test Student there. Then you never need to login and logout to test things. You do your site administration or course building in one browser, and just flick to the other browser to see what the Student sees! Very quick and easy, and works really well when you are developing content quickly and checking