You can create a centroid point within the polygons using feature to point If you have fairly regular polygons, then the default location should work, otherwise a 'label point' location will be chosen ensuring placement within the polygon. You can then choose an appropriate marker for the point and/or label it. There are other options, but I find that actually having the central locations as their own featureclass helpful in some types of analysis. If this is purely for cartographic purposes, there are other ways as well.