Physics's don't know that only negative charges move. We can create ion currents on demand in many environments. We do know that the current flowing in a metal wire is negative charges in motion.

As for how to determine that, you do a Hall effect measurement.

The measurement works by subjecting a current in a relatively wide bar to a magnetic field perpendicular to both the current and the width of the bar and then measuring the potential difference across the width. In this era of turn-key precision voltage measurements is easy enough to do in a high school laboratory if the students can follow the underlying arguments.

Physics's don't know that only negatively-charged particles move. We can create ion currents on demand in many environments. We do know that the current flowing in a metal wire is negatively charged particles in motion.

As for how to determine that, you do a Hall effect measurement.

The measurement works by subjecting a current in a relatively wide bar to a magnetic field perpendicular to both the current and the width of the bar and then measuring the potential difference across the width. In this era of turn-key precision voltage measurements is easy enough to do in a high school laboratory if the students can follow the underlying arguments.