General:
* Sharp: Intellectually quick and perceptive
* Astute: Having or showing an ability to accurately assess situations or people
* Keen: Having or showing a strong interest or desire; eager
* Resourceful: Able to find quick and clever ways to overcome difficulties
* Observant: Paying close attention to details
* Intrigued: Aroused by curiosity or interest
* Methodical: Done in a careful and organized way
* Dedicated: Devoted to a task or purpose
* Tenacious: Determined; not easily deterred
* Persuasive: Able to convince others to do or believe something
Hard-boiled:
* Cynical: Believing that people are motivated by selfishness
* World-weary: Tired of life and its problems
* Grizzled: Having a weathered, seasoned appearance
* Rugged: Tough and strong
* Uncompromising: Refusing to yield or compromise
* Unsentimental: Not influenced by emotions or feelings
* Stoic: Calm and unemotional in the face of pain or suffering
Eccentric:
* Quirky: Having peculiar or unusual habits or characteristics
* Eccentric: Strange or unusual, especially in a way that is amusing rather than annoying
* Unorthodox: Not conforming to accepted standards or practices
* Whimsical: Acting or behaving in a capricious manner
* Idiosyncratic: Relating to the distinctive characteristics of a person or thing
Other:
* Brilliant: Exceptionally intelligent
* Ingenious: Clever, original, and inventive
* Brilliant: Exceptionally intelligent
* Stealthy: Moving or acting in a secret or sneaky way
* Discreet: Careful about what one says or does; tactful
* Dogged: Persistent and determined
* Intuitive: Having or characterized by a quick and ready insight
* Analytical: Relating to or using analysis
* Pragmatic: Dealing with things realistically and practically
You can choose the adjectives that best suit the specific detective you are describing, and you can also combine them to create more nuanced and interesting descriptions.