Most likely carbon dating coupled with at what level in the strata the amber was discovered.
I was under the impression carbon dating isn't reliable past 50,000 years or so.
As I understand it, C-14 half life is about 5,700 yrs. It seems that that would make it impossible to date something over 11,400 yrs old using this method.
I've no reason to doubt that it is 20 mil years old, give or take, but I'm not sure how they would have determined that number.
Most likely not. Carbon-14 goes back only about 50,000 years.