On most old homes the tile is placed directly on a cement layer (not plaster!) with metal or wood lathe underneath and you can place new tile directly on this, as it takes the place of cement board... or vice versa.
Check to see if the wall under the tile is cement. Cement will be much harder than plaster. Try scratching it with a nail -- cement will scratch very little if at all. You need to be certain that it is cement and not plaster before you tile.