According to Tagawa Gardens' website, seasonal needle shed (also called needle drop) shows up on the oldest needles - the ones usually on the part of the bark closest to the trunk. Old needles are also found further out on a branch, however.
Conifers drop those inner needles primarily because they’re old and heavily shaded. Their job is done. The tree will grow new needles to do their work.
Older faded needles may hang on until a strong wind blows them off. You can grab a clump of these interior needles and give a tug. If they come off in a clump, but near-by green needles stay put, you’re almost certainly dealing with a shed.If you do see any of those symptoms or suspect other issues, grab you smart phone, take your best wide- and tight-shots of the problem and bring those pictures to Dick’s Corner at Tagawa for a diagnosis. In the meantime, take a deep breath. Your conifers may simply be doing what conifers do.
Here's the full article with excellent pictures showing exactly what needle shed looks like on a conifer.
Photo and article credit: Tagawa Gardens