How can churches know if they are being effective at making disciples?

Many churches are measuring the wrong things. We measure things like attendance and giving, but we should be looking at more fundamental things like anger, contempt, honesty, and the degree to which people are under the thumb of their lusts. Those things can be counted, but not as easily as offerings.

Why don't more churches gauge these qualities among their people?

First of all, many leaders don't want to measure these qualities because what they usually discover is not worth bragging about. We'd rather focus on institutional measures of success. Secondly, we must have people who are willing to be assessed in these ways. And finally, we need the right tools to measure spiritual formation. There are some good tools available like Randy Frazee's Christian Life Profile and Monvee.com, which John Ortberg likes.