In short: "100% cashmere" only says that nothing else is mixed into the yarn. It does not say how good the cashmere is. Quality depends mostly on how thin the fibre is (finer is softer) and how long it is (longer pills less), so ask the seller for both.
What the label says
Only what the yarn is made of. It is like saying a table is 100% wood: is it oak, or the cheapest board you can find?
What decides quality
Thin is soft. Cashmere is generally fibre thinner than about 19 micron, and the finest grades are under about 15.5 micron. A human hair is about 70 micron.
Long is strong. Longer fibres, from about 36 mm, stay together in the yarn. Short fibres stick out and form little balls, called pilling.
Three questions to ask a seller
How fine and how long is the fibre? Where is it made? How does it look after a season of wear? A maker who is proud of the quality answers with numbers.
Next: Is cashmere worth it?