How to detect surface-wave loss in microstrip antennas using HFSS?

It was observed that HFSS tends to overestimate the radiation efficiency of microstrip patch antennas. I strongly doubt that this overestimation is because that HFSS regards surface-wave power as radiation instead of loss. Is there any method to isolate the surface-wave power from the space-wave power in HFSS? Any comment is appreciated.