Detect a point is inside a polygon with google maps

 
// Ray Cast Point in Polygon extension for Google Maps
// Thanks to App Delegate Inc

// For API V2
GPolygon.prototype.contains = function(latLng) {
    var inPolygon = false;
    // Exclude points outside of bounds as there is no way they are in the polygon
    var bounds = this.getBounds();
    if  (bounds!=null && bounds.contiansLatLng) {
        // Raycast point in polygon method
        var numPoints = this.getVertexCount();
        var j = numPoints - 1;
        var P = latLng;
        for (var i = 0; i < numPoints; i++) {
            var P1 = this.getVertex(i);
            var P2 = this.getVertex(j);
            if (P1.lng()<P.lng() && P.lng()<=P2.lng()) {
                if ( (P2.lng()-P1.lng())*(P.lat()-P1.lat()) < (P.lng()-P1.lng())*(P2.lat()-P1.lat()) ) {
                    inPolygon = !inPolygon;
                }
            }
            j = i;
        }
    }
    return inPolygon;
}

// For API V3
google.maps.Polygon.prototype.contains = function(latLng) {
    var inPolygon = false;
    var path = this.getPath();
    if (path) {
        // Raycast point in polygon method
        var numPoints = path.getLength();
        var j = numPoints - 1;
        var P = latLng;
        for (var i = 0; i < numPoints; i++) {
            var P1 = path.getAt(i);
            var P2 = path.getAt(j);
            if (P1.lng()<P.lng() && P.lng()<=P2.lng()) {
                if ( (P2.lng()-P1.lng())*(P.lat()-P1.lat()) < (P.lng()-P1.lng())*(P2.lat()-P1.lat()) ) {
                    inPolygon = !inPolygon;
                }
            }
            j = i;
        }
    }
    return inPolygon;
}