I was posed this question in the last Bible Study I taught. How can you really know if your ministry is successful? It can be tricky to answer. First, you can look at it quantitatively. How much has my ministry grown? I was X amount of people last year and now I am Y amount of people this year. As long as Y is greater than X, I have a successful ministry. I don't think many church leaders would phrase it like that but a capitalistic society trains us to look at numbers as a determining factor. Well, butts in seats do not necessarily mean success in spirituality. Second, you can look at how 'pure' your ministry is. Many churches are content with stagnant numbers as long as the leadership and everyone else is preaching the 'pure' doctrines. Here, there is more of a qualitative assessment than quantitative.
But I believe the Bible poses one measuring stick, The Fruit of the Spirit (Gal 5:22-23, Eph 5:9). If the Holy Spirit of God is actively at work in your ministry and the hearts of people are being changed and transformed then you will see evidence of this in their lives (James 2:20, 2:26). They will make wise decisions. They will impact people around them. They will have great children, families and marriages. The Fruit of the Sprit will manifest itself.
If you teach some off the wall doctrine just to increase your membership, well the Holy Spirit is not going to manifest itself. Conversely, if you teach a 'pure' doctrine with no fruit or evidence of the Fruit of the Holy Spirit, then how can you claim you have a successful ministry?