The Ignore Test
DC's require your attention to exist.
Other dreamers do not.

Therefore, a simple test is this: If you think a person in a dream may be real, as in another dreamer with their own mind, ignore them. Just turn you back on them, and if they are a DC, they will have vanished.

If it is another dreamer, they will still be there.

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Traits of DC's vs. Dreamers

Intellect
Another way to tell is DC's are kind of idiotic. They tend to talk in circles, nonsense, or even total gibberish. If they say anything intelligent, it sounds like something you would say, or something you would expect them to say.

Another dreamer (if they are inexperienced) will usually say, "This is my dream! Get out of my dream!" because they, like you, were raised with the silly silly notion that dreams are all in your head. I always laugh when they say that. Other dreamers will have conversations with you. They will often surprise you with their intellect. Another common thing they may say is, "No, you're not real, I'm real."

Energy

A real dreamer has a different feel. Imagine the energy of a character on a TV show. Now imagine someone is in the room with you. They are silent, but they are there.

Appearance

A dreamer may have a very detailed appearance. A DC will generally have a much more simple appearance.