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
- Any amount of alcohol greater than 3.4 ounces must be packed in
- 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.