5 years later-version:
Today, there are JS libraries for you, if you don't want to get into the nitty gritty of the different methods described on this page.
On of these is https://github.com/hubspot/offline. It checks for the connectivity of a pre-defined URI, by default your favicon. It automatically detects when the user's connectivity has been reestablished and provides neat events like up
and down
, which you can bind to in order to update your UI.