Look at who plays those opening frequently. If someone responds with the Slav in 80% of their games against 1. d4, and they're a GM, you can be sure they're a leading expert -- regardless of if they write on it or not.
Opening Specialists
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Who are the leading experts on openings such as Nimzo Indian, Slav defense, Semi-Slav defense, Sicilian-Taimanov, etc? It would be nice to know as it would help me in my study of master games.