The main form of evaluation should be a listening test with multiple naive listeners. But there are other ways to evaluate, and potentially to improve, your voice.
How to save a synthesised waveform
festival> (set! myutt (SayText "Hello world.")) festival> (utt.save.wave myutt "myutt.wav" 'riff)
and of course you can change myutt.wav to any other name you like (but always keep the .wav suffix).
Use the forum to find out more about tools for running an evaluation.