I can only think of one for sure sign that you designed your web page well.

When the users do not even notice the design of the web page then you know you designed it well.

This needs some explaining I guess. If the user has to think about the design of a site at all you did a bad job. This is true even if the user thinks, "I like this design". Your design should be so nice and smooth that everything just flows and the user does not even notice the layout or colors or anything visual about the site, except the content.

also...

When a user goes to your web site everything should be where the user expects it to be. In the back of all of our minds when we go to find a button on a web page we all expect it to be in a certain place. This is true even during your first visit to the site. In the history of the Internet certain standards have developed, side toolbars, top toolbars and such. If a user goes to look for a button and can't find it you did a bad job designing your site.