Trigonometry .... use distance meter and vertical angle to determine altitude... Sine Angle * Len Hyp = vertical height.
A survey total station could do this rather easily depending on the materials covering the plane and reflectance. Accident reconstruction teams already use this kind of equipment. It could be measured from a distance so you might not even know they were there. They would be trying pretty hard to make this work.
They could intercept the telemetry and read the values being sent to you. This would be pretty damning to you because you would know the altitudes and they would have the same information as you.
I think that the only time this is really going to come into play is when an aircraft is hit several thousand feet in the air.