In Plessy v. Ferguson, how did railcar attendants know Plessy was black?

Question

He was 7/8 white and looked white.

Answer 1

The Citizens' Committee to Test the Constitutionality of the Separate Car Act was working in cooperation with the East Louisiana Railroad Co., whose officials believed that the 1890 law was costing the railroad money. So the rail conductor knew exactly who to ask to leave, despite the fact that Plessy looked white.

Answer 2

That is why he was picked.... they had to ask.