Over the weekend I found myself using another from my family. When you 'know' about someone but have never met them and couldn't recognise them, the phrase
"I wouldn't know 'em from a bar of soap!"
was used when I was a child. At around the time I went to uni, I started hearing the alternativ
"I wouldn't know her/him if s/he stood up in my soup!"

Soap sold by the length (a bar, about 12" long) was being replaced by the much less working class "cake" (often packaged separately as 1 cake/package) at around that time.

Cheers, Rowan