Dr. Robert Godfrey, former president of Westminster Seminary in California, and current fellow at Ligonier Ministries, has written a very helpful article about recognizing whether a church is faithful or not. His article is based upon the theology of the Protestant Reformation, particularly the thinking of the Reformed and Lutheran traditions. Here is a sampling:
Among the Reformed churches, eventually three marks were identified: faithful preaching of the Word, faithful administration of the sacraments, and faithful exercise of discipline. In focusing on the marks of the church, the Reformers were not saying that all a good church needs to have are the marks of the church. They focused on the marks because the marks make the true church recognizable. The church of Christ has many more characteristics than the three marks. But these characteristics—we might mention prayer, fellowship, devotion—are not so easy to observe. The marks are important because they display the faithfulness of the church.
Read the read of the article at Ligonier Ministries