Q: How do you evaluate/test accuracy of Text-to-Speech (TTS) models? As the title implies... 
For instance, for Machine Translation, we have BLEU.
For categorization, we have categorical crossentropy, for binary categorization, we have binary crossentropy, etc. etc.
For machine learning methods, I'm not sure what we can use to measure accuracy of TTS models. 

A: It is better to start exploring such a complex topic like TTS with a textbook. The book by Paul Taylor is good, it covers speech evaluation too
There are basically two approaches - subjective evaluation and objective evaluation. For subjective evaluation the most popular evaluation metric is MOS (mean opinion score test), but there are other more complicated tests like MUSHRA
For objective evaluations the most popular test is simple MCD test (mel cepstral distortion), but there are more advanced ones. For more details see
Speech synthesis evaluation Sébastien Le Maguer
There are also more advanced machine learning methods for synthesis quality, see this paper:
Towards Signal-Based Instrumental Quality Diagnosis for Text-to-Speech Systems by Tiago H. Falk, Student Member, IEEE, and Sebastian Möller
also
Non-intrusive Quality Assessment of Synthesized Speech using Spectral Features and Support Vector Regression by Meet H. Soni, Hemant A. Patil
