When does one cross the line from acknowledging obvious human differences to being an outright racist?

Would joking about the physical features or general behaviour of another race do it? Is someone who cracks a mean racist joke automatically a racist or just an insensitive jerk who may or may not be drunk at the time?

What if you have racist thoughts but hates yourself for having them? Are you then still a racist? What if you know someone who is a racist and keeps mouthing off without you saying anything, what does that make you?

Is it simply as defining it as to how and in what manner, words and ways of expression regarding race differences come out of one’s mouth?