I believe Meteor.userId is initially null, but is then filled in with the id of the currently logged-in user. If there is no logged-in user, it stays null.
The question is, how do I know if there is a logged-in user, vs. when the client is waiting to find out if there is a logged in user? I’d like my app to know asap.
The reason I ask is that I want to put up a sign-in screen if the user is not logged in. If there is a logged-in user, I’d like to avoid them seeing the sign-in screen while the client is waiting to find out that they are logged in. 
To put it another way: let’s say I had a “loading” message on screen, waiting to find out if there is a logged-in user. It so happens that in this case, there is no logged-in user. How would my app know that there is no logged-in user?