In order to manage fermentation we have to monitor the Brix and the temperature on a daily basis. Brix is the measurement we use for sugar and we all know what temperature is. 
As fermentation progresses the yeast use up the sugar, decreasing the Brix and producing three byproducts; Carbon Dioxide, alcohol and heat. If a fermentation gets too hot it can 'blow off' the the fruity-ness, cause off flavors or aromas and even cause a stuck ferment. The temperature can be regulated by the use of glycol jackets on tanks or cooler location for barrels. Both of which are kept cooler in the beginning of fermentation to promote a slow leisurely pace for the gluttonous yeast. This will give a better chance of the yeast eating all the sugar then dying. Otherwise they gorge themselves then die before the sugar is all gone. Also whites are kept cooler throughout simply because they have more delicate fruit to lose if overheated. 
I measure the Brix with a hydrometer and of course I use a thermometer for temps. At bigger wineries I have had the opportunity to use one of these DMA do-hickies, pictured below, you push down the plunger at the top, it sucks a very small amount of juice up and gives a read out of both Brix & temp (it does a lot of other really cool stuff but I wont get into that now). These are very expensive, even just to have them calibrated is a lot of money so I do not have one at my disposal right now. 
But remember noses and taste-buds are the most important tool we have to determine any issues that a fermentation may be having. I always stick my nose in my sample and give it a taste as well.
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