Sumit Kukreti BSE calculation in VASP will let you calculate the dielectric function of your system of study. With GW calculation you will get the quasiparticle gap of the material as well.

Once you have completed GW and BSE calculations extract dielectric function. VASPWIKI website has some script to extract dielectric function. It will give you a file with energy in one column and oscillator strength of particular transition in the second column. In this information you can find that some optical transitions are lower than the quasiparticle gap. These transitions create bound excitons. Among these, some of them will have their oscillator strength zero and some others are having comparatively larger values. The former ones are dark excitons and the latter are bright excitons.

Thank You @ Sruthil. Yes, I extracted the imaginary part of the dielectric function using the script given in the vaspwiki example.

There are some values that are 0.002 ..like so you mean all these values correspond to dark excitons? or anything other.