This may be a naive question, but, how do we know that the mass we measure for a black hole is equivalent to the things that went into the black Hole? We can’t

This may be a naive question, but, how do we know that the mass we measure for a black hole is equivalent to the things that went into the black Hole? We can’t have any observational data on what went into them in aggregate - is it not possible that the mass of black holes that we estimate is incorrect in some fundamental way? Or just that e.g. energy that enters them also becomes mass?