decisive
You can call a person who knows what they want assertive, decisive, or self-assured.
vulcanologist.
you call them a computer tech
Educated.
A person who knows three languages is typically referred to as a trilingual.
A person who knows the dictionary by heart is often referred to as a lexicomane or a lexiconophilist.
a animal wisperer
An automotive technician.
because no one knows
an accessory after the fact may be what you are looking for
Counterfeit money ...
Popular, social, a people person etc.
Someone who knows the number you want or directory enquiries.