[mlc9852]

How do you suppose he came to the conclusion it was 20 million years old?

[Prime Choice]

How do you suppose he came to the conclusion it was 20 million years old?

Most likely carbon dating coupled with at what level in the strata the amber was discovered.

[bigmac0707]

It doesn't say which dating methods were used here, only that the date was determined by extracting blood-- and I'm not sure how one would date something based on organic materials. Unless the spider was radioactive of course. (In that case, the movie ends up really bad for the humans)

[Alter Kaker]

How do you suppose he came to the conclusion it was 20 million years old?

I think he probably used scientific dating techniques. Not having read the study, I have no idea which techniques he used, but if you're interested, your local university library probably subscribes to Paleontology and I'm sure the study goes over its methodology quite sufficiently.

[Joe 6-pack]

"How do you suppose he came to the conclusion it was 20 million years old?"

Because it was 19,999,997 years old when he found it, and that was three years ago.

[chaos_5]

Carbon 14
Its radio active and created by all living carbon-based organisms. The blood would contain this because it is organic, unlike most fossil which are little more than mineral imprints of what once was.
This is a typical dating method.