Thermionic electron emission in a vacuum tube would let you distinguish between electron and element: the electrons are liberated much more easily then the source material and result in current flow through the circuit.

This lets you build diodes as a result, so assignment of electrical direction based on that phenomenon would get it correct.

EDIT: in fact with a cathode ray tube you can literally visualise charge direction from looking at a foil wheel being spun in a vacuum: https://m.youtube.com/watch?v=K2G6M3cYJZs