Out of the box I am not aware of a quick and easy way to see which
specific site  collection belongs to which content  DB. You can see
which Virtual server belongs to which Content DB but can not drill
down to see top-level site collections. There is a third party tool
(free) that will let you see this.

It is quite a useful tool called SharePoint explorer you can check it
out here:
www.ontolica.com/.../SharePointExplorer
Home.html

This tool will allow you to drill down into your site collections and
see as to what content DB it belongs to.

Locate your site  collection in the SITES table in the config database.
Using the associated databaseId, check the databases table in the config
database to see the content  database name.

The utilities in the WSSUserUtilies will also produce a report that will show you where site  collections are located.