If you want to check whether a browser has HTML 5 geolocation support enabled and display a warning message to the user if it is not, then this function should help you out.

function displaynosupportmessage()
{
    if(!navigator.geolocation)
    {
        $('#map_canvas’).text('Your browser does not support HTML 5 geolocation. ');
    }
}

In this example I check to see if the navigator.geolocation method is available. If not then I use a jQuery command to update the div that would normally house the Google Map by adding warning text for the user.