Either they've got distinctive styles (Darjeeling steeps too long, etc), or Rosehip has stolen sips from everyone's cups while they were brewing for themselves and can remember those.
she smelled the traces of the one brewed it in the cup?
even sharper than police dog
U~m, this is, let's see...P-Pekoe-san brewed it... What is it desu wa!?That's rightAssam-sama brewed this... It's, it's u~m!This here was brewed by Darjeeling-sama~ Ah~ I wonder what it is!Oh wow, how scary...Just why does this girl know who brewed what?Taste of RosehipIt hasn't come to me yet, but~ This, this is~...!Rosehip's tea-tasting challenge!I'll guess everything desu wa!!Best of luck...