"While all members are periodically involved in evaluating how the group is performing, the good leader will be constantly evaluating and taking corrective actions as necessary."
Once your group gets rolling, it's easy to just let it be. But I find it helpful to take the pulse of the group periodically. To do this I use various evaluation tools.
The first is the most informal, and hopefully the most obvious. My co-leader and I try to stay in touch with the members and casually ask them how they're enjoying the group. What do they like? What don't they like? How can we make it more effective in their lives? If you show interest and are careful not to take offense at their comments, people are willing to tell you their needs. If you actually act on their suggestions, they'll begin to trust that you care about them and will tell you even more.
Second, after we've been meeting for a few weeks I do a short written evaluation with two or three questions. This allows the more reluctant members to give anonymous feedback. Usually I'll ask, "What two things do you like best about our group?" and, "What two things would you like to see improved?" It's short, simple, and takes five minutes. It says, "I care. Your opinions are important to me." If the group is continuing beyond 12 weeks, I may do this type of survey again when we're about two-thirds of the way through. For an ongoing group, I'll do it every few months.
Finally, at the end of the group or at the end of each study (usually 8 to 12 weeks), I do a more comprehensive evaluation. I ask for input on the quality of the study, the group's organization, the leaders and co-leaders, and anything else I want to know about. Most important, ...