Wrong. It is written right into the criminal code that a reasonable idea is inadequate. The law actually requires you to take positive, reasonable steps to confirm age. You cannot base your decision on appearances, or vague social circumstances.
The code explicitly requires that you take "all reasonable steps" to ascertain age, that is positive action on your part aimed at discovering age, and everything that you could reasonably do. There is case law that if you have taken several reasonable steps you can stop--so presumably, if you have just attended her 18th birthday party with all her friends and family, you don't have to ask for ID.
"Gleaning" it from appearances specifically does not pass the laugh test as everybody knows there are 13 year olds who look older than they are. The law requires you to check.
What is not going to be acceptable is that she told you she was 16 or you heard that from her pimp, or some other equally unreliable source.
In summary, if you are fucking relative strangers who are young and potentially under age then yes indeed the law more or less requires you to ask for ID. To rely on social circumstnaces you would have to have hard facts to go on, like you know that she attends a college or university (likely 16), you've seen her driving a car (has a drivers license), and so on. Note you need MULTIPLE factors.
If you hardly know someone they could be lying about being in college, driving a car, etc., so you should ask for ID.