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A field strength meter is a good instrument to use for this, but unfortunately the sensitivity of most units made for ham or CB work is too low for part 15 AM use. You might get a usable reading if the FSM is close to the antenna but in so doing it may detune the antenna causing false results.
Keep this in mind when shopping for a meter. No harm in trying, but I would ask for a return guarantee in case it doesn’t work in this application.