sharp-eyed lookout

Translations
осторожный наблюдатель
The sharp-eyed lookout spotted the approaching ship long before anyone else.
Осторожный наблюдатель заметил приближающийся корабль задолго до остальных.
Definitions
A person who is very observant and attentive, especially when watching for something specific or keeping watch for danger or important events.