I found that if you don't buy your own bassinet, the one the game gives you might give away the baby's occult-- you might just get a generic one, but you might also get one that matches the baby's occult.
And I just did that for the baby my sims had, spellcaster mother and vampire father. The game gave me a spellcaster bassinet, but for a couple of seconds after he first appeared, there was a couple of vampire icons over him! Also, he's quite light-skined, but his thumbnail is very dark. He might be a hybrid!
Update: As a toddler, he's definitely a spellcaster. If Maxis wanted it to remain a mystery until they become Teens, maybe they shouldn't use the unique Needs icon on the toolbar...
Still very interested to see if he also starts becoming a vampire when he reaches Teen though.