> What assurances do we have that none of it is
> transferred up to the honey supers?

None at all, if you overfeed, and your colonies
do not "burn all the HFCS up" raising brood.

> At what level is HFCS actually discernable
> in honey?

Very very very very very very very low levels.

A "carbon isotope ratio" test can detect HFCS in honey.
(I'd have to check the latest lab equipment catalogs
to give an accurate detection level, but if anyone wants
to check, the kewl way I think is state-of-the-art is via
"Infrared Laser Spectroscopy".  For all I know, that
method could be "soooo 20 minutes ago" by now.)

The difference depends upon the specific photosynthetic
process used by each plant.  Honey "should" be made from
"C3" (Calvin cycle) plants rather than "C4" (Hatch-Slack
cycle).

The ratio of C-13 to C-12 is what varies between C3
and C4 type plants.

Corn is a "C4" plant, while all (to my non-botanist knowledge)
honey plants are "C3".  Bad news, sugar beets are "C3" type
plants, but not to worry, there are other tests to find
sugar-beet adulterated honey.

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