shrewd observer

Translations
проницательный наблюдатель
As a shrewd observer, she quickly noticed the changes in the market.
Будучи проницательным наблюдателем, она быстро заметила изменения на рынке.
Definitions
A person who is astute and perceptive, able to notice and understand things quickly and accurately, often using this insight to make informed judgments or decisions.