All successful requests return some data. For example, a successful GET will return the data requested. A successful POST for inserting a record returns the ID of the newly inserted record.
You can use these clues to generate a success message for your user.
Any unsuccessful request will have a in the json response a key called "code" which tells you the error code. By checking for the existence of the "code" key in the response, you can assume a failure and generate a message for that.