Obsessive Compulsive Disorder=a mental illness resulting in persistant illogical fears and ideas, and often, particular habits created and followed out so as to avoid the fears and anxiety they cause.
you can't ever tell for sure unless they are tested, but some common practices of OCD are doing something over and over, being overprotective, seeming anxious, making things perfect, etc. I have a friend who to me I am positive she's OCD because she just has weird little things about her and needs everything to be perfect and in her control. If you need more help, just visit this website-http://funadvice.com/r/bjkrrf2tb3t
You don't know. Every single person on this earch has some sort of behaviour which seems OCD, it is how we put structure into our lives imo. The best way to find out if you really have OCD is to speak to a doctor. They'll be able to advise you the best.
If you're a therapist or doctor, then you diagnose them. Otherwise, you ask the person whether they've been diagnosed. There's not actually a test. It's just like the rest of the mental disorders, it needs to be diagnosed.
They have their stuff arranged neatly or color coded etc.