how to validate a generative model?

Soledad Villar (NYU) has created a deep-ish network that can generate fake APOGEE spectra. They look convincing! Now her question to me today was: How do we validate a generative model? How do we know that these generated spectra are reasonable or sensible? In astrophysics we have no ground truth. All we can think of so far is looking at whether it is possible to do parameter estimation using standard stellar spectroscopy pipelines on these stars, and whether different parts of the spectra deliver similar stellar parameters (or as similar as do different parts of the spectra of real stars, which is not always that similar!). We can also compare to The Cannon, which is also a generative model (though not a deep one!).