Note: Before reading further, first consider Mt 7:3; 18:15-17, and Eph 4:15 (and other biblical texts concerning proper church discipline).
Every once in a while a fellow believer will say something, something theological, that doesn't sit right with me. When that happens the first thing I do is tell myself to not be casually agreeable...you know, that involuntary head nod that politely tells a person you're tracking with them.
Second, I genuinely listen. Afterall, I can't tell you how many times I've been dead wrong about something. Sometimes it's my own error. Other times, it's the bad theology I learned growing up or at, say, seminary. Whatever the case, I can't learn if I don't listen. In this regard, I want to be like Apollos as I have plenty of Priscilla and Aquila's in my life (see Acts 18:24-26).
Third, I dialogue. And best I can, I try to make counter arguments using only the Scriptures. I do this for two reasons. First, it keeps the conversation from turning into an opinion-fest. Second, if the person becomes offended, their offense is with the Bible, not with me.
By this time the issue is usually resolved. If not, I simply say, "Can you give me some time to think about this? To study the Scriptures regarding this topic?" Then, I go do just that...I study and, if necessary, seek wise counsel. I don't study to show up the other person. I don't study to gather loads of arsenal. Instead, I study to present myself before God as one approved (2 Tim 2:15). Then, in some appropriate way, I present my findings.
If the issue is still unresolved, one of two things happens. If the issue concerns a non-essential theology, I just live in the tension, pray, and continue to study. If the issue is an essential theology, I proceed cautiously, carefully following the Scripture's directives (Mt 18:15-17). Then, I follow the Spirit's leading. Sometimes God directs me to let the whole thing play out. Other times, God directs me to distance myself from the situation. And every once in a while, God directs me to rebuke (rebuking, by the way, doesn't need to be mean-spirited).