This is a request for a 'HasIndex(string indexName)' or 'ContainsIndex(string indexName)' helper.

Unless I'm missing something, to find out if an index exists on a collection, one must:

var query = await myCollection.Indexes.ListAsync();
var bsonIndexes = await query.ToListAsync();
var indexNames = bsonIndexes .Select(i => i.Values); // or some variation
return indexNames.Contains("blah");