You know, brothers and sisters, that our visit to you was not without results. We had previously suffered and been treated outrageously in Philippi, as you know, but with the help of our God we dared to tell you his gospel in the face of strong opposition. For the appeal we make does not spring from error or impure motives, nor are we trying to trick you. On the contrary, we speak as those approved by God to be entrusted with the gospel. We are not trying to please people but God, who tests our hearts. You know we never used flattery, nor did we put on a mask to cover up greed—God is our witness. We were not looking for praise from people, not from you or anyone else, even though as apostles of Christ we could have asserted our authority. Instead, we were like young children among you.
Just as a nursing mother cares for her children, so we cared for you. Because we loved you so much, we were delighted to share with you not only the gospel of God but our lives as well. Surely you remember, brothers and sisters, our toil and hardship; we worked night and day in order not to be a burden to anyone while we preached the gospel of God to you. You are witnesses, and so is God, of how holy, righteous and blameless we were among you who believed. For you know that we dealt with each of you as a father deals with his own children, encouraging, comforting and urging you to live lives worthy of God, who calls you into his kingdom and glory. - 1 Thessalonians 2:1-12
There are a lot of reasons to do things in the church and many of them are bad. Paul lists a number of them here: error in teaching, impure motives, deception, getting the approval of some people, getting some people to do what you want, selfish advantage and control of people or the situation.
How could Paul prove to them that they were acting in their best interests? Really, he could not prove it. He had to trust that their compassionate, selfless hard work would speak the truth in parallel with the gospel: "we were delighted to share with you not only the gospel of God but our lives as well." I would say that people are good at detecting fakes: people building relationships tied to an agenda. In today's society, there are plenty of people who will be your friend if you will go along with their plans.
Sometimes my test has become: how do they treat people who aren't their "type"? That's what God did. "But God demonstrates his own love for us in this: While we were still sinners, Christ died for us." (Romans 5:8) How will they know what God is like if they can't see it first in those of his people with skin on.