You can and SHOULD be able to distinguish between a true church, a false church and the various grades in between.
Herman Hoeksema taught that believers ought to join that church which most clearly manifests the three marks, where the preaching, sacraments and church discipline are faithful to God’s Word. He added that those who leave a faithful church for a less faithful church to that extent leave Christ, for Christ’s spiritual presence in a church is marked by faithfulness in its preaching, sacraments and discipline. Hoeksema writes,
… it is my conviction that everyone is conscience-bound to join himself to the purest manifestation of the church of God …
Is it a great sin to unite oneself with a church other than that which is, according to one’s conviction, the purest manifestation of the true church?
It is; for, by doing so one knowingly cooperates with those forces that always tend to the development of the false church. A church need not be wholly false and corrupt to justify separation from its fellowship. Every church is false in the measure that it departs from the Word of God, corrupts the sacraments, and becomes lax or perverse in the exercise of Christian discipline (Ready to Give an Answer, pp. 37-38).