como verificar se existe um campo em um documento específico Mongodb usando C#?

var document = Bundle.Collection().Find(filter); // here is your BsonDocument
try
   {
      document["fieldNameToCheck"] // if field doesn`t exist it throws KeyNotFoundException. If there are nested objects just follow the pattern: document["fieldName"]["fieldNestedToCheck"]
   }
catch (Exception ex) when (ex is KeyNotFoundException)
   {
      // your logic for "the field wasn`t found in the document" case
   } 

var exists = document.Contains("fieldNameToCheck");// if field exists it returns true
// If you need to check the nested fields, you can do as follows:
var nestedExists = document["fieldName"].ToBsonDocument().Contains("fieldNameToCheck"); // or:
var nestedExists = document["fieldName"]["nestedFieldNameNextLevel"].ToBsonDocument().Contains("fieldNameToCheck");  // and so on...      

BsonElement element; // it will contain found element if true for next line
var exists =  document.TryGetElement("fieldNameToCheck", out element); // returns true if element is found