Try White tooth paste if the marks are minor. T-cut can be used if you are careful, I have used Brasso before sucessfully although it can make a bit of a sheene on a mat tile.
If they are bigger/deeper then a glaze rubber can be purchased from pottery suppliers if you are near to Stoke-on-Trent there are lots of places.
Try the toothpaste first as you will be surprised what it will remove.