Redirect To Registration Page

how to force users from a common asp page to the registration page. Example:

Joe blow goes to abc.asp and has a look around. He never registers because there is nothing saying he should register before browsing. I want to force joe blow to register before browsing some pages.

What would an If statement look like in that case? On what object do you perform the check?

I know how to use a redirect statement, I don't know how to perform a check for prior registration or login. I would imagine the check should be the first call of the page?