When talking about determining what a thing is, or how it is defined, one is generally interested in what is called ontology. From WP:
Ontology is the philosophical study of being. More broadly, it studies concepts that directly relate to being, in particular becoming, existence, reality, as well as the basic categories of being and their relations. Traditionally listed as a part of the major branch of philosophy known as metaphysics, ontology often deals with questions concerning what entities exist or may be said to exist and how such entities may be grouped, related within a hierarchy, and subdivided according to similarities and differences.
When discussing how this is accomplished, the discipline is known as metaontology, and the act of choosing one ontological theory over another is known as one's ontological committment which often refers to the character of one's ontological theory.
When one has a theory of what things exist, one engages in epistemology to decide if statements about the existence of objects is knowledge versus belief. Knowledge is how we can tell whether or not an object that seems to exists really exists.
An example might help to clear things up:
If you look at the world and you think there might be invisible beings, you are engaging in an ontological act because you are declaring the possibility of the existence of something. You might then attempt to define the being as invisible, capable of moving objects, and being friendly towards humans. Now, is this belief or knowledge? Belief is tentative, and knowledge is more certain. It's not knowledge unless you have some epistemological means for justifying the belief. You might pick up a book on science and realize that you need to design field and laboratory tests and use naturalistic observation. After testing and thinking, you decide that these beings don't exist. You now have changed the ontological committment of your theory of beings to exclude invisible ones. You might go on to define any claims of invisible beings as supernatural beings, and define any claims of beings that are visible as natural beings. These definitions change your ontological theory about intelligent beings.