If it's not in a challenge or dynasty file, I add them to the family (by pressing ctrl-shift-c then typing "testingcheatsenabled true", shift clicking on the sim in question and selecting "Add to active family") and have a look at their family tree.
If it's in a challenge or dynasty file, at the beginning of the game it's easy to tell - with Angeline, the only Alto's in the town are Vita, Nick and Holly, and Holly was still a teen when Angeline was born. This meant that Nick and Vita had to be the parents.
Further down the track, it's hard to find out unless you have someone in the family that's related to other townies. For example, in my dynasty, my sim married Cameron Bachelor who was the son of Bella Bachelor and Mortimer Goth. I went back through the Goth side of the family tree and discovered that Gunther (Mortimer's dad) had a step-child with the Bunch family. This meant I could see all of the Bachelor, Goth and Bunch family trees from then on (and let me say, the Bunches have bred like rabbits in my game!).