Would the TSA open up a Growler of homebrew in a checked bag?

If they feel it is indicated, yes. There is no specific formula that TSA uses or advertises to determine when to open suspicious items. While there may be guidelines, by the nature of the job TSA agents have discretion to inspect items they feel may pose a risk.

In all likelihood they will probably not open your growler, but it is impossible for anyone to guarantee that the item would not be opened and inspected.

The TSA blog makes specific mention of rules related to carrying alcahol in both checked and carry-on baggage, for those interested: TSA Travel Tips Tuesday - Traveling With Alcoholic Beverages

Checked Baggage

• Any amount of alcohol greater than 3.4 ounces must be packed in checked baggage.
• Alcoholic beverages with more than 70% alcohol content (140 proof), including 95% grain alcohol and 150 proof rum, cannot be packed in checked luggage.
• Travelers may take up to five liters of alcohol with alcohol content between 24% and 70% per person as checked luggage if it’s packaged in a sealable bottle or flask.
• Alcoholic beverages with less than 24% alcohol content are not subject to hazardous materials regulations.

As 5 liters is equal to around 169 fluid ounces, you would be allowed to bring 2 growlers into checked baggage, if the beverage is between 24% and 70% alcohol. Most beer and cider is much less than that, so 1-2 growlers would certainly not run afoul of these rules.

However, absolutely no guarantee is given by any agency or airport in the world, as far as I know, that they won't open something they deem questionable.

So, what can you do about it? Well, the easiest method I suggest is to use your own form of "seal", such as tape, and wrap it around the top of the bottle -assuming you don't just happen to have some tamper-evident adhesive seals with you (which I am think is a safe assumption). The goal is just to make it obvious if someone opened it, tearing or loosening the tape.

While this won't prevent anyone from opening it, at least it will be obvious that it can't be opened in secret, and when you get your bag back you will have a pretty good chance of knowing if someone opened the bottle. Which will let you know that you either just wasted the money, or at the very least gives you a chance to drink it immediately before it goes flat and stale.

It will probably help if the bottle is labeled prominently by the brewery, so it's a bit less fishy looking, but I doubt it would matter that much. Either the agents involved have seen it before or don't care, or they'll think your bag is suspicious and ruin your brew.

A lot will depend on where you are flying from and the TSA agent's familiarity with beer / growlers.

If the TSA agent does not know what a growler is, then to them it is a large stainless steel container filled with an unknown liquid, which could be flammable, could be harmless. I would attach a label to it indicating what is in it, preferably a beer label or tag from the brewery, so they at least have an idea what is supposed to be in there.

Other than that there is no way to know for sure if they will open it during inspection or not. TSA doesn't make its guidelines known for a very good reason.

Since it is stainless steel, it will not show as hollow on X-ray. So they are likely to open the bag. Then what they do with the growler depends on whether they know what it is.

64oz of liquid, I would think it likely they would have a look because it's going to simply be a blob on their x-ray--and in my experience bags containing blobs like that get opened.