Need a good A/C service tech more than anything but one somewhat familiar with NSX's should be able to diagnose it faster. They'll be able to use the flourescent dyes or have an electronic detector.

If its 134a then personally i'd go get a recharge kit from Autozone or wherever and see if it works or how long it lasts, it's an older car so probably needs it anyways and may have just been overlooked at the dealer it was bought at