Goodgulf:
Back to the parent trap, there's a scene where one of the girls is being sent upstairs to change, with a few sold pats on her bottom to hurry her along. Unlike the staged spanking scenes of today, you can see Hailey's neither cheek roll with the smacks. Followed by a brief debate about spanking them until they say who was who.
Wasn't it Maureen O"Hara (spanko-famous for being spanked by John Wayne in "McLintock") who planted her palm smartly on Hayley Mills' derriere in that scene?
IIRC Brian Keith, portraying Maureen's once and future husband, nixed the idea of spanking the twins (to get them to admit who was whom) by saying "You don't spank twelve-year-olds," which I considered rather incogruous with reality--at that time, there were undoubtedly at least hundreds of thousands of RL twelve-year-olds across the U.S.A. who were subject to parental paddlings.
However, realistically the parents should've been able to figure out which twin was which in fairly short order if they'd tried--a decent parent can certainly tell a child whom he/she has raised and lived with all of her life from one whom he/she hasn't, I'm figuring.
Of course, that would've ruined the film's rather far-fetched (IMHO) plotline...

--C.K.