Yesterday I looked at meteor sources to see if there was anything like that. I could not find the associated flag or anything else ...
I think you will have two options:
Implement a heartbeat on the client and server for each connected user. I personally do not really like this idea, as this can lead to an increase in the number of intervals performed on your server.
Use sockjs server to get open sockets . Meteor.default_server.stream_server.all_sockets() returns an array with all open sockets. You can use one period of time to look for changes (or better you will listen to the changes of the sockjs server itself, there is a register method that can be useful), map open sockets to your users and use to push it to your clients. Each client knows its socket ID, so matching does not have to be hard.
I haven't implemented it yet, so these are just ideas to get started.