also: アームチェアーディテクティブ
noun
armchair detective
A person who solves crimes or mysteries from home, using reasoning and available information rather than fieldwork. Borrowed directly from English and used in similar contexts.
彼は現場に行かずに事件を解決するアームチェアディテクティブだ。
He is an armchair detective who solves cases without going to the scene.
アームチェアディテクティブのように、ネット
Like an armchair detective, I deduce things using only online information.