# Observables in covariant formulation of electromagnetism

I feel some confusion regarding how and what can be measured in covariant formulation of electromagnetism. In Why is the electromagnetic four-potential $A_{\mu}$ not an observable? it seems that the 4-potential can not be measured. What are the observables- is the 4-current and/or the electromagnetic field tensor measurable and how?

Feels like the engineering side of things are harder to find information on...