You measure how many people with different backgrounds (measured by a variety of metrics) gain entry to the pipelines that are recognized as the most common ways to gain power, wealth and prestige in a society.

You don't require that they all actually gain power, wealth and prestige (since that measures something else, which could be equally important or not, depending on your perspective).

If the only way to become a SCOTUS justice is to get into one of 2 or 3 law schools, and only people with a narrowly defined profile ever get into such schools, you pretty clearly do not have equality of opportunity. You can establish this even though in reality almost nobody ever becomes a SCOTUS justice.