You can determine if the current request is being made by an authenticated user or not using if(Request.IsAuthenticated)

The web is not a stateful environment, there is no easy way to tell whether particular users are logged in or not except in the context of a web request.

We do have a feature in the Site Statistics that tells who is online but it is not precise, it assumes a user is online if he has made a request in the past 20 minutes which is the default session timeout. 

Hope it helps,

Joe

You need to learn C#, then you would know the not operator is !

so

if(!Request.IsAuthenticated)
{

Code to run if the user is not authenticated...
} 

Hope it helps,

Joe