By their age of course.
But we are talking about extraordinary evidence being required of a man known as a sexual predator. It is not "extraordinary" for someone like Bill not to bother asking the age of mature looking young woman.
All they need to be is "ready".
I asked a few people who did not see the movie how old they think someone like Jody Foster in this picture would be...
The guess range went from 16 to 18 ( she was 12 then ).
Bill Clinton looks to me like somebody whose sexual appetite does not discriminate based on "age" ( Monica was 22 ). So, I won't use the word "extraordinary" when it comes to Bill's sexual appetites.
@SirLinksALot IMO,the perfect example of what you are writing about is the girl that got Rob Lowe in all that trouble. Ruined his career for over a decade. He had sex with a girl that turned out to only be 15 years old,and everybody was justifiably horrified.
What wasn't published at first was he met the girl at a late night Dim Party fundraiser,she was wearing professional makeup,a sexy adult dress that was low-cut,and was drinking a martini with a woman that turned out to be her mother when Lowe was introduced to her.
It's after 10 PM at night,he meets this smoking hot female that looks to be 25-30 years old,is attending an adult party,drinking hard liquor,and flirting with men in their 30's and 40's. Lowe was also drinking. What was he supposed to have done,asked to see her fake ID? IMHO,it is not unreasonable given those circumstances for ANY adult male to take what is being thrown in his face without asking questions.