Historically there are three distinguishing marks that identify a true church. They are:
1) The preaching of the Word
2) Administration of the sacraments/ordinances (The Lord's Table and baptism)
3) Administration of church discipline
If any of these three are lacking, then you don't really have a true church. However, how do you know if a church is fulfilling these things properly? I have found a web site that seeks to answer that question. The details of these three marks are expanded into "9 Marks," which just so happens to be the name of their website. :-)
http://www.9marks.org
Mark Dever leads this ministry. Here he describes the following nine marks of a "healthy" church. If you read these, you will see the historical three marks contained in these points. Yet, there is a clarity offered here that is very worthwhile. I intend to spend some time browsing through this site. The nine marks are:
1. Expositional Preaching
2. Biblical Theology
3. Biblical Understanding of the Good News
4. Biblical Understanding of Conversion
5. Biblical Understanding of Evangelism
6. Biblical Understanding of Membership
7. Biblical Church Discipline
8. Promotion of Christian Discipleship and Growth
9. Biblical Understanding of Leadership