How to measure output voltage and current of a VFD?
Not so easily. It's a really "dirty" signal if you look at it on an oscilloscope. You would need something like a power meter with fairly wide bandwidth, and to measure both current and voltage together (ie. simultaneous samples at a high sample rate, likely with anti-aliasing) and the meter would have to multiply and integrate the result . 

Such meters certainly exist, but maybe they are not so affordable.