An astute observer is someone who pays close attention to their surroundings, notices the details, and understands things quickly and accurately. Full definition
When astute observers decided that the market was overpriced, they withdrew their funds. (religion-online.org)
Lawyers started breaking their «paragraphs» twice or so per page, making it apparent to any moderately astute observer that they weren't really writing paragraphs at all. (abajournal.com)
It, of course, led to plenty of questions from more astute observers. (techradar.com)