Understanding of the object concept in Object Oriented Programming (OOP) is obviously the center of the paradigm. Many educators and researchers explored students’ difficulties and developed teaching materials targeted at this central concept. The paper presents a diagnostic tool we developed that aims to reveal students’ perception and understanding about the current object, referring to it by the this annotation. Proper conceptualization of this indicates an understanding of objects in general, and involves aspects of memory allocation and programming approaches. The tool contains five questions, each devoted to covering different aspects in various frameworks, such as: using this in constructors, using this as a visible parameter, using this in inheritance, or making necessary changes in transition from a non-static context that uses this to a static context. The questionnaire combines closed questions with a request to explain the answers and open questions. In the paper we present the purpose of each question, and address what it comes to examine. The diagnostic tool is based on known educational approaches: Bloom’s taxonomy, assessment for, as, and of learning and learning from errors. The tool can be used by educators at high school or academic levels as a teaching tool, as a base for discussions, or as an evaluation tool. A short report on the use of the tool with different populations, including high school teachers, is presented. The paper uses Java as the programming language, but it easily can be translated to other OOP languages.