vulcanologist.
You can call a person who knows what they want assertive, decisive, or self-assured.
you call them a computer tech
A person who knows three languages is typically referred to as a trilingual.
Educated.
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
A very talented person. Multi-lingual.Most probably more talented than YOU.
You would call this person a child. By the time a child reaches puberty, he or she knows.
mechanic car-bore-ator