UV/Vis spectrocopy on a powder sample using an Ulbricht Integrating Sphere...?

You should verify it by doing experiments. If your crystal is a semiconductor (which I suppose it is) then you can do resistivity measurements as a function of temperature. That will give you the band gap. There are several other spectroscopic methods as well. Just read any standard book on semiconductors.

I think UV-Vis diffuse reflection spectrum can test the practical energy gap.

With a simple experience of optical absortion. Or as Dr. Chatterji suggest, by resistivity measurements. Depends of the characteristic of your sample.

Tauc's plot gives an idea about band gap from optical absorption spectra.