The answer is, very carefully. It depends on part on how close you are to that person. If you are not a close friend, talking to them may be very difficult. If you know them well, you could ask them point blank, “Why are you so critical of others?” But I would not do that unless I knew them well. It’s also important that you have three or four good examples of what you mean. A general statement like, “I think you are too critical” means much less than, “When you spoke harshly to Dean Jones about how he teaches the 5th graders, it made him want to give up the class altogether.”
And pray before you say anything. Be sure you take to heart what Jesus said in Matthew 7:1-5 about taking the log out of your own eye. Judge yourself first before you say anything to the person in your church.
For more on this, see Judge Not!