Mmmm
Two excellent choices there!
I would treat the oxtail exactly how you would any other slow cooked beef stew type recipe for the slow cooker.
Personally, I would make the oxtail stew, let it cool, fish out the bones and let the meat fall off, then make a second brew using the bones for bone broth.
As for the chicken livers... love 'em. I pan fry them with lots of butter, a dash of red wine, masses of garlic and a sprinkle of Rosemary.
The way to tell if they are cooked is that they go from soft and flobboly to firm. If in doubt, cut one in half. If the texture is consistent all the way through, they are cooked, even if they still have a slight pink tinge. If they are still red and soft in the middle, then cook them a little longer. You want to avoid over-cooking, because then they will go hard, dry and grainy.
BUT
even if you do over cook them, all is not lost. Just blend them up with a dollop of cream cheese to form a puree and chill.
et voila! pâté