Detect the visible_changed event on the StreetViewPanorama associated with your Map object. You can get the panorama from the map by calling its getStreetView() method and bind the handler to that object’s event. You will have to test the StreetViewPanorama‘s visibility by calling its getVisible() method.
For instance:
var map = new google.maps.Map(document.getElementById("theMap"), {streetViewControl: true});
var thePanorama = map.getStreetView();
google.maps.event.addListener(thePanorama, 'visible_changed', function() {
if (thePanorama.getVisible()) {
// Display your street view visible UI
} else {
// Display your original UI
}
});
See the events section of the StreetViewPanorama Object Documentation for more events you can listen for on this object.