Electric motors, especially three-phase induction motors, consume a significant portion of industrial electricity, making their efficiency critical for energy conservation. Traditional efficiency assessments require intrusive, labor-intensive laboratory tests, limiting in-situ evaluations. We propose a non-intrusive, heuristic optimization-based method compliant with IEEE Std-112 A/F1 to simultaneously estimate equivalent circuit parameters and in-situ efficiency using only field-measurable electrical quantities. Eight meta-heuristic algorithms were applied across six motors of varying power ratings and load conditions, with results validated against standard test data. The method achieved mean absolute errors below 0.7% at full load, demonstrating high accuracy and stability. This approach enables reliable, low-cost, and continuous efficiency monitoring under real operating conditions, offering practical benefits for industrial energy management and motor performance assessment.