I am assuming that you are extending the ItemizedOverlay<T> object to manage your Overlays. Well, this class provides a hook which gives the ability to handle touches to any of the OverlayItem's in your ItemizedOverlay<T>.
public boolean onTap(GeoPoint point, MapView mapView)
GeoPoint point -- the location on the map of the 'tapped' Overlay.
MapView mapView -- the MapView on which the Overlay resides.
If you are not extending ItemizedOverlay<T>, could you let me know what you are using, and I will see if I can help.