Dear Ajay Kumar Mahato,

Fundamentally, we can find by performing absorption spectroscopy of the material. A direct bandgap material will absorb the light of wavelength equal to its band-gap whereas an in-direct bandgap material will not. On the other hand,

Theoretically the energy gap can be determined by a first-principles calculation within the Density Functional Theory (DFT).

Warm Regards

Dhanu Chettri

Experimentally you can check with Photoluminescence signal. Indirect bandgap semiconductors don't give PL signal while direct ones give strong PL signal.