Android users with a Google account added to their phone should know that the color of their signal bars will change depending on the status of their connection to Google. Android keeps a connection open to Google for various reasons (push notifications, e-mail sync, etc.). If this connection goes away for some reason (like, say, if your provider doesn't have any bandwidth left for you), the connection bars will change to grey.

It's not fool proof, but it's better than nothing.