Flight Instructor reported they were informed that a person on the ground had reportedly observed the instructor's aircraft descend below 500 feet AGL during training maneuvers.

Narrative

My commercial student was practicing ground reference maneuvers along [a] road. She had completed Eights on Pylons and was practicing S-turns along a road; as the winds were strong and it was a good time to practice.Today I received a call from the FSDO saying that we had been reported below 500ft by a person on the ground. It's not clear if we were below 500 ft or were too close horizontally.Since the pivotal altitude is between 600 and 1000 agl and our instruments only read msl at the nearest airport it; is possible that we were too low or too close at some point especially if the reported point was on a hill. I am always conscious of CFIT; and always keep in mind where to land should an emergency arise; in this case there was lots of open land and quiet roads available.All we can do is eyeball our height above the ground and in this case we've been informed that we were too low.In future I will train my students in more remote areas; and try the maneuvers at a higher pivotal altitude.