Does "Cook's distance" tell us the outlier? How many way to find the outlier?
For cook's distance, which level is the cut off of outlier?

The criterion can vary. For a clear beginning explanation you can look up "Cook's distance" in Wikipedia and look at the section entitled "Detecting Highly Influential Observations".
The conservative option they discuss is basically a hypothesis test, so your threshold value is based on the significance level (i.e. value of alpha) you choose. 
HTH,
Jack
