Theologians often talk about the marks of the True Church (unity, sanctity, catholicity, and apostolicity), however in re-reading the Scriptures, I've found a via negativa to answer this question that is actually quite meaningful.
Rather than simply look for positives, what are the negative attributes that Holy Writ ascribes to the True Church.
1. St. Paul asks the rhetorical question in Romans "What shall we say then? Shall we continue in sin, that grace may abound?" (6:1). That's a question people should ask if you're preaching the Word rightly. Would anyone ask that of Aquinas or even Augustine? Obviously not. In Augustinian and Thomist soteriology, grace is lost the second one willfully sins. There is no room for sin and grace together, an utter repudiation of simul iust et peccator.
2. Do philosophers think your message is stupid? In Acts 17, St. Paul was mocked and called a babbler for his preaching. His message he called "foolishness" to those who were perishing and the Greeks (1 Cor 1). Is your Gospel foolish or is it sophistical and philosophically brilliant?
3. Beware false teachers and false Christs. CONSTANTLY throughout the New Testament Christ and the Apostles are warning about false teachers. If they asserted the indefectibility of the Church (Matt. 16) it was perhaps the most limitted type one could imagine (think Noah, or St. Athanasius). Otherwise, why would they warn so often about it?
4. Winston Churchill once said having enemies was a good quality because it means you stand for something. Christ said his true disciples would be hated by the world. This probably doesn't help anyone, as each Christian group is hated by any number of the other Christian groups let alone the world hating all Christians. Nonetheless hatred seems to be a mark of the true church.
Perhaps another useful ecclesiological tool would be to see the marks of a false church in the Bible.