You can (at least, last I checked) query a connected player's
cl_connectmethod convar to see if it reports the
serverbrowser_favorites string, which indicates that they connected via the favorites tab in the server browser. There's no way to query an arbitrary Steam ID to see if they've added a server to their favorites.
Both the
Player Analytics and
Favorite Connections plugin (and probably others) can report that a player connected through via favorites; it's also not too difficult to implement it yourself.
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