What does it mean to be "real"? Can you list the necessary traits of things that are real? How can we tell what is real and what is not?

The question of what is "real" or what sort of entities exist is the object of a branch of philosophy known as ontologies. Most philosophical systems are said to possess ontologies in the sense of making different claims about the sort of entities that populate their universes.

Philosophers differ as to whether reality is determined by sense perception or by abstract reasoning. Empiricists such as Hume argue that only those things that can be perceived by our sense are real. They take what first appears to be a common sense position that we can judge what is real by the evidence of our own senses. This view, though, is far more nuanced among philosophers than it might seem on the surface. Several debates ensue from this position.

The first has to do with qualities. If I perceive many different objects which have in common certain visual similarities, which I describe as "red", does this mean that "red" has some sort of independent existence or is it mere an artifact of my mental processes of perceiving similarities among objects? Next, there is concern about whether the senses are reliable. Someone who has a high fever, a brain tumor, is drunk, or is taking drugs might have disordered perceptions. We could say that the pink elephant perceived by someone with disordered perceptions is not "real", but then we become embroiled in a potentially circular argument in which we judge a person's perceptions disordered because they perceive things that are not real, but claim that the perceptions of those people are disordered on the grounds that they claim to perceive unreal things.

Another position is idealism (sometimes, confusingly for nonspecialists, called "realism") which presumes that since the world we perceive by our senses is constantly changing and is perceived differently by different people, it is not real, but only a pale and illusory imitation of an eternal unchanging reality. This position is held in certain forms by Plato and many Buddhist theologians.

Another strand of philosophical thought argues that our raw experiences are mediated through certain mental constructs shaping our perceptions, such as what Kant termed the "synthetic a priori intuitions". Thus we see the world as existing in space and time and events as having causal relationships to one another due to the structure of our minds. 

Each philosophical system, therefore, suggests different ways of telling what is or is not real and proposes a different list of traits of things that are real or not real.