Using Smell to Determine Degree of Roast

There are subtle aroma shifts that occur as coffee is roasted and you will quickly become aware of the noticeable differences. At lighter roast levels you can detect delicate aromas as steam escapes the bean, but as you go darker this steam turns to smoke as you begin to incinerate the sugars and volatile organic compounds locked inside.  

Here are differences in aroma you’ll notice while roasting:

Yellowing:  As the green coffee begins to change color to a pale yellow (this occurs roughly half way through the roasting process) you will notice a grassy aroma.
First Crack:  The beans begin to release steam that has a fragrant, sweet aroma.
City to City+:  When a roast is at this stage the aroma should be sweet and complex.  There might be a slight maltiness and a sugary hint.  
Full City to Full City+: The aroma begins to have a slight burnt sugar and toasted nut character.  Since the cell structure of the bean is breaking down you will notice more smoke.
Light Vienna, Vienna, and Light French:  Once you have reached this stage there will be a strong burning smell and much more smoke.  The sweetness is long gone as you are in the process of burning the coffee bean.  You better stop the roast if you see and smell large amounts of smoke coming from your roaster.

Smell of Roasted, Ground Coffee

Aroma also comes in to play when assessing your roasted coffee as you prepare to brew it (and even after it is brewed!).  One of the best ways to determine roast level is by grinding your roasted coffee and deeply inhaling the aroma from the dry grounds.  Everyone loves to do this!  It is also very informative.  As discussed in the section on sight, the aroma will vary, as well as the color. Here are the differences in aroma you’ll notice when assessing dry grounds and brewed coffee:
City and City+ roasts will have very sweet, slightly malty and possibly floral characteristics.  
Full City and Full City+ roasts will have more bittersweet, chocolaty and ripe fruit character.  
Light Vienna and French roasts will be bitter and possibly burnt smelling.   

*These are generalizations that can vary widely with the origin of the coffee.