I guess you can sniff the airlock but you get some nasty smells off even a healthy fermentation.
If you are careful about temps, pitch enough healthy happy yeast you don't really have to monitor them. They have been turning sugar into alcohol for millions of years and at this point have a pretty good handle on how that works.
We just try to guide them through control of environment (temp, pH, nutrient availability, etc), starting population, and basic external genetic phenotype. Pick the right yeast, pitch the correct amount at a reasonable temp into an environment stocked with a good balance of macro and micro nutrients and you should be golden.
you can monitor gravity as it changes and flavor to get an idea of what beer at various points tastes like. some of those points are pretty gross so don't get discouraged by rotten egg, overly bitter, fake butter, green apple or the variety of other things you might smell and taste in the early stages. be patient and have faith in our little unicellular friends. They want to make beer for you.