If I smoke some bud, I can't tell you what kind it is. But I can usually tell you what it isn't.
I can tell the difference between an indica and a sativa for example, or I could say "That's definitely not Train Wreck" or "That doesn't taste like Haze".
Beyond that, there's no way to say "That's definitely White Widow", because it might the white shark, or the white smurf, or who knows what.