Color of Smoke
The color of your smoke will tell you a lot about your fire. A cleaning burning fire will produce an exhaust that is either a light thin blue or even totally clear. That's the smoke you want. Watch your fire and you'll see it go through phases.
Dark, gray smoke – When you first light up and the fuel is just beginning to catch, the particles in the smoke are large enough that they absorb light (Franklin 94).
Heavy white smoke – This is what many beginners mistake for "Look at all that good smoke!". The particles are smaller, but they still have creosote, impure carbon, and soot that will give your food that harsh, over-smoked flavor. This can also happen when the oxygen supply is abruptly reduced, such as; choking the fire by closing the air intake too much, too fast.
Thin blue – Once your fire is approaching full combustion, it will turn from a heavy white to a lighter white and then to a thin blue color. The wood will be breaking down into the smallest components that create that sweet smoke, and these smaller sized particles only reflect the blue wavelengths of the color spectrum (Franklin 94).
Clear – This is not smoke at this point, rather it is wood vapor existing your smoker. But clear doesn't mean that nothing is happening. Franklin notes that nearly all the compounds needed to color, preserve, and smoke your meat are contained in that invisible exhaust (91).
Velocity of Smoke
You want your smoke to be moving. Franklin writes that rolling hot air cooks food faster and more evenly than warm air that is just meandering (and cooling) in your cooking chamber (49). In his classes, Christopher Prieto, Pitmaster of Prime BBQ in North Carolina, advises watching your exhaust to see how your smoke is behaving. It should be leaving the smoke stack with some velocity to it. Given relatively still wind conditions, a good clean smoke will continue to rise into the air. Exhaust from a struggling fire will hang in the air or even start to fall after leaving the smoker chimney stack.
Smell of Smoke
Give your smoke a sniff. Cup your hand over the smoke stack's exhaust and draw some of the exhaust towards your nose to get a good whiff. A fire that's choked down without enough oxygen will have noxious components that will smell harsh and bitter. A clean fire with "good" smoke will smell sweet, like the pleasant smell of a fireplace burning off in the distance on a cool Winter day.
So, when you are smoking, start noticing the color, velocity, and aroma of your exhaust and relate it to how well your fire is burning at the time. If you make a practice of this, before long, you will be able to tell how your cooker is running by just watching the smoke.