Figuring out the down fill power of a used garment is not likely to be easy.
If the Federal content tag, often combined with the garment care tag, contains the down:Feather content, you can make a guess.
Typically these numbers are as much as 90 or 95%%down : 10% or 5% feathers, for the best quality down with a fill power approximating 800 cu. in./oz.
More feathers gives you a lower fill power.
High quality down with high fill power is expensive.
The typical down quilt sold in a department store is somewhere around 500 fill power to maybe 650 fill power. Some may be even lower, and a custom high quality down quilt may be higher.
I have a 650 fill power down vest with an 80:20 ratio.
You could also try to learn how to build a down fill measuring instrument by modifying the test methods used in the industry. But, I wouldn't want to get that involved myself.
Or, you could search used sleeping bags that get posted here for a used but high quality bag, such as Western Mountaineering or other that originally had high quality down fill. There is an article here on how to convert a down sleeping bag into a quilt, if that is what you ultimately want.