An object that can be seen is visible.
Something that can be heard is audible.
What's a similar word to indicate that something is smellable?
Both olfactory and olfactive have this meaning, but both words also have the more common meaning of emitting a smell, so they wouldn't be useful.
The best choice would be olfactible. Wiktionary includes it, but as far as I can tell, they're the only ones.
Olfactible (“having an odor; capable of being smelled”) seems relevant.
You may find the wordnik list for odorific of interest; it includes about five dozen “smelling words”, the first few being odoriferous, malodorous, heavenly, redolent, putrid, delectable, stagnant, fragrant, smelly, noisome, noxious, fetid, aromatic, dank, musty, moldy, fusty.
(Odourous in the UK) (Same in the UK, see below)
Scented if the smell was added (Strawberry-scented envelopes, but not scented strawberries) or smelly if the smell is pungent or unpleasant (smelly cheese can be good, smelly socks aren't).
An object that can be smelled might be called "fragrant".
Smellable is reported in the NOAD as derivative of smell, and I imagine it means able to be smelled, in the same way calculable means "able to be measured or assessed."
English syntax allows us to stick smell and able together.
Something gives off an odor (or perhaps doesn't) but for some reason, we cannot detect it. Perhaps it is too far!
I see a flower in a yard across the street, but it isn't smellable from here, though it is undoubtedly odorous, odoriferous, fragrant, and smelly. It can be smelled, just not in this situation.
The word you want is odoriferous (sometimes shortened to odiferous): "Having an odour or fragrance."
A synonym given on the Wiktionary link is odorous.
Aromatic is another word for this:
1 : of, relating to, or having aroma:
a : fragrant
b : having a strong smell
"Odiferous", which seems to be a contraction of "odoriferous" means "yielding an odor" per Merriam Webster (odoriferous)
Odorable. (Not to be confused with 'adorable')
Meaning 'Able to be smelled. Perceptible to the sense of smell'.
The word chemosensory seems to fit what's being asked for.