By smelling a perfume on somebody's cloth on road, how can you know what is its brand name or its ingredients?

Sometimes, while I am walking down the roads, I smell this lovely fragrance coming from somebody's dress. But due to street-protocol, I have never asked them that what was the brand they use. I searched in the perfume shops, but so many perfumes are there stacked together, that you cannot distinguish one fragrance from the other. Besides, they will not spray on you every individual brands as a sample. Because, if every customer wants sample, then all their perfume bottles will have gone evaporated within weeks. So, it is also not possible to smell every individual perfume bottle. And it is also not possible to buy every costly perfumes one by one, because it will take much time for these bottles to be empty, before I can buy the next brands. So, what should be done? Should I ask the person on the road directly that what's the brand? Will it be fair?