How to detect new Machine like Facebook?

Question

I am actually trying to detect a person, using a computer, Just like facebook. Whenever you sign into facebook using different machine, it detects the new machine and asks you t

Answer 1

I checked my facebook cookies and (contrary to @Mattis) i found a cookie called c_user, i think that is the actual cookie that holds your id in a 15 digit decimal number, also another persistent cookie called datr that stores a 21 digit string with _ at the end

I hope i helped