In short: click on every state and check to see if it's a core of anyone; and learn who gain what, when. There's also a list (I think) on the wiki.
(As a general rule, all of a country's starting non-colonial territory is core territory, and cores on the territory of other countries is rare - at the start of the game, only Bolivia, the Soviets, India, and the two main Chinas have cores on territory that isn't theirs.)
For Germany specifically:
- Germany starts with cores on all of it starting territory.
- If the Anschluss happens peacefully, Germany gains cores on all of Austria. I don't remember exactly how this event works, but I think it only applies to Austrian-owned cores/claims.
- If the transfer of Memel happens peacefully, Germany gets a core on it.
- If the transfer of Danzig does not happen peacefully, Germany gets a core on it.
For France:
- All of Metropolitan France (plus Corsica) start as French cores, and the three or four states making up modern-day Algeria can also become cores via national focus.