Open up your character window, and hover your mouse over your titles. An infobox should now appear, showing you the name of the title, the succession law and the line of succession.
I do not know a way to change the laws other than the one for your primary title, however. :/
EDIT:
Ok, I think I understand now. I believe it the core of the answer is whether or not a title is a de jure vassal to one of your greater titles. The succession law of a title will always be determined by the greatest title you hold in that "realm". If you have titles in other "realms", those will be determined separately.
- For example, if you are King of Ireland and the Duke of Munster
(which is de jure part of Ireland), then the duchy will inherit the
succession law of the Kingdom of Ireland.
- However, if you are also the Duke of East Anglia (de jure part of
England), then that duchy will have its own succession laws.
- If you also hold the title of King of England, then the duchy of East
Anglia will inherit its succession laws from England (which are
separate from the succession laws of Ireland, even if you hold both
titles).
- Now, if you also happen to be Emperor of Brittania, then I assume
all the titles from within that realm inherit the succession laws from your primary
title.
- Should you hold titles which are not de jure part of Brittania (say,
King of Poland), then those will have their own laws, and the chain will start over from count to emperor, depending on your greatest title within that "realm".
PS: I know my use of the word 'realm' may be a bit different from the one used in game, but I could not find one that fit. What I mean by "realm", is a collection of titles which are de jure connected under one greater title which you control. I hope my intentions comes across despite this.
Alright, so this question got me curious enough to dive into the game files to try to set up a situation like what you've experienced to see what the game will do.
So here we have King Guinea of the mighty and ancient house of O'Pig:
Now King Guinea O'Pig has lived a long, long life, recently celebrating his 100th birthday. In that time, he's had two children, his eldest Gwenny O'Pig:
And his youngest, Porky O'Pig:
At the party, some say addled by advanced age, others by years of blasphemous, heretical lies, he made the declaration that his daughter Gwenny, a woman of all things, should have equal standing in terms of inheritance.
On his inevitable and swift trip to meet the reaper, Gwenny became the new ruler of Laigin, herself the mother of two children, an older daughter and a younger son.
Strangely enough, Gwenny rather liked the idea of holding onto power, and of allowing her daughter to inherit just the same as she did, despite having never held to her father's bizarre, unorthodox religious delusions.
As such, she made the decision to keep laws as they were, even if they contradicted the "norm" for her faith.
So there you have it. If this little experiment is anything to go by, it does indeed appear that even on the inheritance of a non-heretic character, a succession law allowed only by being of that heresy will stick. I may just have to see if switching Gwenny and Porky's ages has any effect.
Edit: After trying it out with the two children's age's flipped, I can confirm that even if Porky inherits rather than Gwenny, the succession law remains unchanged.
Best Answer
It's complicated :)
If you are a count, then the succession laws for your county will apply.
If you are a duke, then the succession laws for counties don't matter to you and instead the succession laws of the duchy are applied.
If you are a king, same thing.
If you are a king, however, you can hold multiple kingships and each one has a different succession law.
If you are an emperor then that one is separate from your duchy/kingdom.
In any case, it seems that you were a count/earl and then become a duke. First you had one succession law then another one.
Gavelkind is fine for small realms. If you change to elective then you have to start giving land to your sons. If you don't have spare counties/duchies then it's not great.
It's not a problem that you lose a title to a sibling. What you can do is hatch a plot to take it back or just revoke it. Since you're probably in a small realm, the negative hit to opinion won't matter much. If you're in a bigger realm then you should go elective or primogeniture.