Check If The Login Is Valid?

After I send a post request to login to a site (using httpwebrequest), how do I check if the login is valid?

One way to do it is to read the whole html using

dim reader as new streamreader(response.getresponsestream) dim str as string = reader.readtoend and then check the html for any unique string (e.g. "log out"), to tell if the login is successful.

But reading the entire html is too long, so how do I shorten the process?

I tried

Dim str as string = response.getresponseheader("Set-Cookie")
But str is a blank string.