How to check socket is alive (connected) in socket.io with multiple nodes and socket.io-redis

I am using socket.io with multiple nodes, socket.io-redis and nginx. I follow this guide: http://ift.tt/1qZBKxJ

How can I check socket is alive (connected) with socketid

I tried namespace.connected[socketid], it only work for current process.

Anyone can help me? Thanks for advance