How would someone actually measure and EER in the field?

Measure temperature and humidity of air in vs out, and air flow. Calculate
actual heat pumped per hour from consequent latent and sensible heats.
Meter power at panel to get actual energy used per hour. Divide to get
heat per energy, which is efficiency.

Or estimate refrigerant mass flow from compressor pressure differential,
displacement, and volumetric efficiency, to get heat pumped per hour, given
no gas out or liquid in.