You don't need good eyes as much as the ability to read the signs. The big clue is the wrench flats. Adding flats is an extra manufacturing operation with added cost. Manufacturers are cheap, and pinch every penny and fraction of a penny. So if a part has wrench flats, it's because it needs to be turned.

Then the vestigial thread peaking out was the clincher.

So if you're wondering how to get into a Chinese Puzzle box of a bike part, look for visible clues and ask yourself what they mean.