Which species carry charge between contacting insulators
In plain words
When two insulators touch, charge moves between them, even though insulators have no free electrons. Experiments support electrons, ions such as hydroxide from adsorbed water, or torn-off bits of material, depending on the system.
Precise statement
For polymer-polymer, polymer-oxide and identical-material contacts, with surface charge densities up to about $8\,\mathrm{statC}/\mathrm{cm}^2$ (the air-breakdown limit), determine the fraction of transferred charge carried by electrons, ions ($\mathrm{H}+$, $\mathrm{OH}-$, other) and transferred material. Identification must be chemical or spectroscopic (x-ray photoelectron spectroscopy, mass spectrometry, isotope labeling) with a closed charge balance. An answer is a quantified carrier partition for a set of benchmark pairs.
What would settle it
An experiment in which the transferred charge equals, within error, the chemically identified transferred species for each benchmark pair.