If this person you feel an urge to criticize is not yet your close friend, you'll need to spend some time with him. Find out everything that's good about him, and go out of your way to help him out. Eventually, a real friendship will develop
Also, you'll need to ensure that this person has the same knowledge, understanding and perspective of right and wrong as you do before you can attack his decisions. If he doesn't, you'll need to spend some time learning and discussing together until you see each other's point of view.
Once the two of you are in the same space in Torah and observance of mitzvot, and he's your good friend to boot, then it's okay to criticize—if necessary. And if you can remember what there was to criticize.