Отговорът на @im1dermike вече не работи за c# драйвер версия 2.0+
Ето алтернатива:
public async Task<bool> CollectionExistsAsync(string collectionName)
{
var filter = new BsonDocument("name", collectionName);
//filter by collection name
var collections = await GetDatabase().ListCollectionsAsync(new ListCollectionsOptions { Filter = filter });
//check for existence
return await collections.AnyAsync();
}