How do I control the quality of a kombucha SCOBY?
If you want your Kombucha be strong, consistent, and to taste like a commercial one, then you need to use the same SCOBY contained in a commercial kombucha. Remember, making Kombucha, just like brewing wine, mead, or beer, is nothing more than glorified yeast herding. You give the SCOBY what it wants to eat, and it does the rest. It's really that simple. You just need the same starter as a commercial company.

Now, many companies spend years perfecting their strains of yeast/bacteria, and trying to raise your own or getting a starter from "some guy" often just won't compare because it is either weak, or it hasn't been fed properly, it contains too much acetalbacter, etc.

So, how do you do it? You use the SCOBY that commercial kombucha makers provide you in the bottle and propagate it. In homebrewing we use a similar procedure called "washing yeast," where you take a commercial beer, drink the majority, and then use the  dregs of yeast at the bottom to ferment your own beer. 

Start with an unpasteurized, raw commercial bottle of Kombucha that you like that has not been sorbated or sulfited. Check the bottle for wisps of sediment in the bottle, indicating that it contains some mother, the more the better.

Then, you prepare food (black tea and sugar), and mix the two. Cover with some cloth, store in a warm (I would guess at 65-70 degrees F based on my experience brewing wine/beer and alsoo purposely making vinegar) and dark place, and let the mother start to grow. It'll take 2-3 feedings before you have a strong mother, but it should form. If it doesn't? Grab another bottle of kombucha and try again.

This link explains the process in better detail.

http://grist.org/food/mother-load-the-secret-to-diy-kombucha/

Once you have the mother, just continue feeding and do whatever you can not to break the cycle of feedings so it doesn't get stressed. You are dealing with living things, and so they will have a tendency to fall into cycles. If you break the cycles, they'll get stressed.