How to validate discussion topic url is valid?

I used the following to check whether an URL for a discussion topic is valid? Both returns false.

 spweb.Url.StartsWith("http://webapp/sites/CommunitySite/Lists/Community Discussion/Topic1");
 spweb.GetFile("http://webapp/sites/CommunitySite/Lists/Community%20Discussion/Topic1").Exists;

Note: Topic1 is the Discussion Title of the discussion posted.