A private investigator, maybe an ex Bureau type or two will be paid to obtain DNA samples for all the Clintons, Web Hubbell, Danny, etc etc and sent to a private lab for analysis.
There are tons of them on the web, and cheap. Do a google search for "paternity Test" and they will show up
How does one find and honest exFBI type? I don't know but consider that a detail that can be overcome with enough money and a little mental effort.
I'm tired of all the whining about this subject.....put up some money and some mental effort or cease complaining.
You could ask G. Gordon Liddy for a referral. :)