The limbs are likely younger than the trunk.

Not to mention they are easily damaged in fires and might give an inaccurate reading.

I had a cat from a kitten for 18 years and once she reached adulthood she looked and acted about the same until the last couple of months before she died. I have one cat now that I am not sure of her age. We guessed one thing, the vet guessed another and her estimate was younger so I kinda went with that.

but I don't know exactly what it is they check for.

I think it's from the teeth but that's about all the details I know.

(FTR both kitties that have owned me, I knew their exact date of birth)

But remember that a female will always lie about her age and weight.