know from something
to know about something. (Used on the eastern seaboard.) Do you know from thermostats? You don't know from anything!
know someone from someone
to tell the difference between one person and another. I don't knowFred from his twin brother. I know Bill from Bob, but I can't tell most identical twins apart.
know something from something
to tell the difference between one thing and another. (Often with a negative.) You don't know a smoked herring from a squid! She didn't know a raven from a crow, and who does?