I am a homebrew Judge and have read a lot about taste and smell along with attending HomBrew Con's on judging. First off, everyone has unique thresholds different flavors and aromas. I have met ppl that can identify diacetyl (off flavor in beer fermentation that smells and taste like butter) with just a small wiff when a bottle is open close to them. For me, I need to look hard for it as I am somewhat blind. So, using this info, I can say by our physiology alone, we can determine differences. Now, one thing to note, it takes time to learn to sense the different flavors and aromas. I for one do not eat bbq on a regular basis (once or twice a week) to be all that in tune with difference in smoke flavor. I also do not have a way to cook identically on the same pit with different wood smoke. What I can perceive from my bbq is subtle differences in flavors. I am usually (not all the time as your sense of smell and taste differ each day and from hour to hour) able to recognize mesquite and hickory on smoked foods.
Beer judging is broken down to aroma, color, flavor and mouth feel. I would say that bbq can be broken down to aroma (smell), color (externally added), flavor (taste and effects perceived in the mouth - dryness, acrid, sweetness).
This is what I perceive and may be different to others:
mesquite
A: strong smoke of burnt wood
C: deep mahagony to black (especially beef)
F: Strong smoke flavor with a burnt wood/charcoal character. There is a unique flavor component that is hard to describe; very dry character
Hickory
A: Strong smoke most similar to that of bacon
C: deep mahagony to black (especially beef)
F: Strong smoke flavor that is bacon like; sweeter than mesquite and not as dry as oak
Oak
A: smells like burning wood
C: red to deep mahogany
F: Taste like pure smoke with little woody character (think red wine that has been matured in barrels); Has a dry character that is more than Hickory and less than Mesquite
Pecan is basically has a subtle hickory character that has a mild nutty quality (I get it more on the Andouille I make than ribs)
Beech/alder
A: Mild clean smoke aroma
C; Alder will add a nice bronze character to seafood and cheese
F: taste the same to me and are subtle like a sweeter oak character with out the woody component
Apple/Cherry
A: Mild, clean smoke aroma that can have a sweet smoke smell
C: Cherry will add a red tingle to meats; apple will add a nice bronze color to cheeses
F: tend to add a clean smoke flavor that has a sweet flavor.
I think if you break down your senses, you might be able to identify some differences.
That is what I have noticed and would like others input.