How do you know if you have ocd?

You don't function : Everyone has some obsessions, and some basic rituals. Keeping your kitchen sink empty, brushing your teeth regularly are a few examples that are healthy. Those keep roaches out of your kitchen and keep you from having to get your teeth drilled. Obsessive compulsive disorder exists when the rituals take over your life and you don't have time for other things, like relationships.

Irrational: In addition to the answer above, i would add that people with OCD know that their obsessions and subsequent compulsions are irrational, yet they cannot stop. Compulsions are performed to stop or satisfy obsessions and serve no useful purpose. Rather, it is debilitating.