The paper cited in the prior post is attached.  (Again, paywalling a 20-year old paper should be a felony).

In the paper no attempt is made to explain how the bees detect "pollen quality", even though they show that low-quality pollen stores prompt more pollen foraging.
What they say about "how do the bees know about protein?" is this:

"Camazine (1993) suggested that pollen foragers obtain information about colony needs during trophallactic contact with nurse bees. The dissemination of protein from the hypopharyngeal
glands of nurse bees to foragers is suggested to act as an inhibitory cue for pollen foraging (Camazine et al. 1998). Although this mechanism was originally designed to explain foraging responses associated with fluctuations in the size (quantity) of a colony’s pollen reserve, it is based on a proteinaceous currency, and may also explain the responses of bees to reductions in pollen quality.
Camazine’s research also supports the hypothesis that foragers determine a colony’s need for protein by “indirect assessment.” Camazine (1993) showed that foragers did not require direct contact with stored pollen to become informed about colony needs, a finding consistent with this experiment (particularly for manipulations of pollen quality)."

So, the bees have a very wacky "commodities trading floor", where the foragers are indiscriminately bringing in whatever they can, and the receivers are the ones making the value judgments.  As with nectar, forager bees are recruited by dances, and make no value judgments on their own.  But relative quality does matter a lot at the colony level, so apple growers must cut their understory of they want the bees to actually work the apples, rather than ignore the apples and work the dandelions.






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