Go for current historical atlases, some nowadays are quite good (especially Autrement's atlases). When not avaible, go for wikipedia : it's not exactly perfect but you have a quite living sub-community of carthographers that try to be as precise as possible.
That say, try to cross-check maps when you can from different sources including some history books or wikipedia articles if you don't have these for the given region or period.
That say, try to cross-check maps when you can from different sources including some history books or wikipedia articles if you don't have these for the given region or period.