edit: also, if you get a question that isn't on the guide anywhere (family questions usually aren't on there), you can just say you can't tell them apart, then talk to them again and they'll have a different quiz.
I've learned a few of them from my villagers through conversation; Biff told me he wanted to be a basketball player the first day he moved in as well as his quote and Harry has mentioned he's the oldest out of 5 children once to me.
It's doable, but a little trickier. Usually the tips I've gotten have been from other villagers - e.g. Chief would tell me about Rosie's catchphrase. I find it easiest to cycle through their questions until I get to the one about their catchphrase, since it's usually some kind of pun about their species and fairly easy to guess based on that. ("be the class clown" for pietro, etc.) Though I save between getting a villager's picture and finding the next house.