How can I determine the voltage insulation rating of magnet wire?
The primary purpose of magnet wire insulation is to provide turn-to-turn or layer-to-layer isolation; as such, the voltages involved are relatively low, regardless of the overall component rating. Therefore the actual voltage rating is usually of little interest. For higher withstand voltages - between the winding and the core or other components - other forms of barrier are normally employed. 

This must be the normal case : if the wire coating had to withstand high voltages, its thickness would increase; then there would be less copper in a given winding space, compromising the efficiency of transformers and motors, or wasting materials by requiring larger bobbins and cores to maintain the same performance. Thus, high voltage copper wire would have a vanishingly small market if it were available.

If you require an air cored inductor with high withstand voltage ratings, I suggest you plan on means other than the varnish coating to achieve those ratings. For example, multi-section bobbins, silicone potting, heatshrink sleeving, nylon mounting bolts and standoffs, or whatever is appropriate to the mechanical situation.