There are three types of odorants:
(1) Molecules that don't exist in nature and are manufactured in the laboratory by chemical means
(2) Odorant elements of essential oils that are synthesized from natural sources
(3) The same odorant elements of essential oils that are synthesized from non-natural sources, such as coal tar derivatives
In the first case, one never knows how a specific molecule will smell until is it synthesized (unless Turin is correct; in which case, one can predict to a certain extent how it will smell beforehand; although, there are a lot of variables, and it's not as simple as it sounds as far I understand with my limited understanding).
In both the second and third cases, perfumers pretty much know how these odorants smell because they have a chance to smell them as synthetically created isolates or nature derived isolates. Even though these isolates smell a certain way by themselves, when they occur naturally in essential oils their scent is the total synergistic effect of their specific odor modified and enhanced by many other odorants. Rose oil has over 300 different odorants that make up its odor profile, but there are about four major odorants (rose alcohols) in terms of volume: geraniol, phenyl ethyl alcohol, citronellol, and nerol. It is the other approxmately 296 odorant constituents of rose oil, even in minute amounts, however, that give a rose oil its complex, inimitable nature specific rosiness. Different kinds of roses also have different proportions of these various major and minor odorants. These minor odorants are more difficult to isolate and to synthesize.
It's one of the cruel ironies of Nature that the easiest odorants to isolate are those that give us the general sense of the odor of a particular thing, say a rose, but all the other myriad odorants that are more difficult to isolate, and in some cases even identify, are what give a rose, for instance, its specific odor that only inimitably exists in toto in Nature.