Even the retail nursery workers seldom know what those rootstocks are. the definition and rootstock cultivars varies with each nursery.
You can look up website of the company in the labels or tags. Oftentimes, they are listed there, like Willits & Newcomb, Four Winds, Citrus Tree Source, C & M, then look them up in the internet and give them a call.
Not even our local nurseries are knowledgeable on the citrus rootstocks. I have meet one really good professional horticulturist at Home Depot who happen to be inspecting the latest shipment, and she can only honestly guess and told me to call the originating nursery telling the particular batch that was shipped.
Sometimes one nusery would outsource from another nursery. Sometimes they simply order from Citrus Tree Source, and then place them in pots and have them grow to marketable size, and then sell off to various other nurseries not keeping track of which rootstocks are which.
The bottomline is that if it is ultra-dwarf, most likely it is Flying Dragon, and the assumption of being ultra dwarf is you keep them in pots forever. I planted an ultra dwarf Calamondin in the ground, you've heard my rant before, and was happy that I don't need to worry about the height, but right now my "ultra-dwarf" is 26 ft tall, and I am not complaining. The same batch of calamondin that I kept in a pot, is still 2 ft tall, indeed an ultra-dwarf but fruiting.
