A well-written essay, article, or even paragraph can reveal much about a writer, such as effort, style, vocabulary, even reading preferences. Good writing takes time and effort, so if you read a piece that is extremely well-written, it is usually indicative that the author of said writing probably worked extensively on his or her writing, editing, proof-reading, and revising. The style and vocabulary used can also be informative as to the writer's literary preferences and knowledge of the subject matter. When writers incorporate informative, detail-rich jargon and subject-specific vocabulary, they develop ethos with their readers, identifying themselves as authorities on their subject.
Whenever I read something that is extremely well-written, first, I am impressed by the author, but I also appreciate that the writer cares so much about their subject to put forth the extra effort to make it amazing.
Good writing takes time, it is not something anyone can suddenly accomplish from sitting down and scraping something up. You can tell their style of writing and how they use their words as well as the position they take up in the argument. You can also tell if they proofread or did well revisions.