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The "what" is usually the Fuselage Reference Line... a line from nose to tail.
Mostly with the horizontal at zero, or the top fuselage longeron, or the floor,...
When the horizontal is set at 0 for the plane in question. do that.
Measure the wing(s) incidence(s) from that reference.
For models, it is usually the case the horizontal being fixed will be at "zero" when the plane is designed.
Scale models OTOH to be scale will do as is mentioned in the "Biplane" thread have other references.. such as the Stearman with its +3 horizontal. That is referenced to a line running thru the prop towards the rudder, paralleling a main fuselage longeron. Just to give the plane designer someplace to start from.
What is important isn't the absolute reference, but the difference in settings, wing to tail, relative to that reference.
The difference is called "decalage"... or "longitudinal dihedral", which is more explanatory.
Harold DeBolt used to set his wings and tails at a positive angle to a horizontal reference line, with the motor mounted on that line with no downthrust. + wing, + tail. This would result in a slight amount of tail-high appearance in flight.
Worked for him.
I built one of his models and set wing and tail at 0.
Worked for me.