Dashboard → My Sites
The My Sites menu item lists all the sites that the user is an administrator of. If you have a different userlevel on another site in the network, it will not be listed.
A drop-down at the top of the page refers to the user's Primary site. This is used as an identifier when doing things like logging in. The user is often redirected to the backend of their primary site on the network.
Each site is listed in the order they are created, and below the name, links to the front end and admin area of each site are provided.