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About the only way to test the sensitivity of an AM receiver with a loop antenna is to supply a known field strength from a test antenna connected to a signal generator. Measurements can be done in the near field of the test antenna, which may be a small inductor. Near field measurements make it possible to do the measurement in a lab rather than in an open field. It is usually not practical for hobbyists to do these kinds of measurements. The measured sensitivity would not be in microvolts, but in microvolts per meter.
If you have access to Terman’s Radio Engineers’ Handbook (1943), the problem is addressed on page 973. Terman refers to a detailed 1941 article in Proc. I.R.E. about testing receivers with loop antennas.