Cole Sear: They see only what they want to see.
Cole Sear: I see dead people.
Cole Sear: Are you a good doctor?
Malcolm Crowe: Well... I used to be. I won an award once. From the Mayor. It had an expensive frame.
Vincent Gray: Do you know why you're afraid when you're alone? I do. I do.
Answer: Cole knew that Malcolm was dead as early as the first few scenes. When he opens his door to the house he doesn't immediately step out. Almost as if he didn't want to go out that day. After he finally muster up the courage, he waits for Malcolm to look away as he hurriedly makes his exit. He also picks up his pace as Malcolm begins to follow him. He knew from the very first time he saw him, and yes he was just as afraid of Malcolm as he was of the other ghost.