"Mr. Redfill, huh? Are you looking to blow a few houses down?" Nat asks, noticing his "Huff & Puff" cigarettes. She knew they weren't real, because she'd become well informed on the subject of counterfeit cigarettes for... uh, casual curiosity. In no way was she professionally informed on how to recognize and produce counterfeit cigarettes, if that's what you're thinking.
"Or are you dressed up as the dangers of smoking, and possibly brooding in a trenchcoat?"