Its done by molecular genetic testing

If looking for specific genetic abnormalities, you do Molecular profiling.

IHC can also gives you a qualitative measure.

But if you are for Homologous repair dysfunction, you test whole HR panel that includes other than BRCA, such h as ATR ATM, BARD1

better to read this: https://www.nature.com/articles/s41525-018-0046-7