Tough to say. This gets at the definition of an "addiction," which isn't all that clear cut to begin with. Many doctors will tell you, for example, that there's no such thing as a "psychological addiction," only a "psychological dependency," since "addiction" by definition requires physical dependency. I'll ignore that argument for the moment.
Here are a few indicators I would look for:
1. Withdrawal: Going without porn causes a significant alteration of your mood, concentration, or physical health (i.e. loss of appetite, lethargy, headaches, etc)
2. Dependency: You must watch or think about porn in order to engage in any sexual activity.
3. Actualization: Your sexual behavior changes to mimic that of porn and/or you begin to carry out pornographic scenarios in real life without prior intent.
These are three big ones that I suspect would separate an "addict" from just the general male population that really, really likes porn.