You need to add an event listener to the map's mousemove event, such as:
google.maps.event.addListener(map,'mousemove', function(mev){
var TILE_SIZE = 256;
var proj = map.getProjection();
var numTiles = 1 << map.getZoom();
var worldCoordinate = proj.fromLatLngToPoint(mev.latLng);
var pixelCoordinate = new google.maps.Point(
worldCoordinate.x * numTiles,
worldCoordinate.y * numTiles);
var tileCoordinate = new google.maps.Point(
Math.floor(pixelCoordinate.x / TILE_SIZE),
Math.floor(pixelCoordinate.y / TILE_SIZE));
debug('TileX:' +tileCoordinate.x+' - TileY:'+tileCoordinate.y);
});
Note that the debug() function is contained in this separate file
The code posted here is largely based on this example from the documentation.