One man and I discussed the issue of those given over to a strong delusion to believe a lie in
2 Thess 2. Paul said this occurs with those who have truth (as said in
Romans 1) but do not have a love for the truth. The brother asked me, "How would we know we are deceived or not? We would be fully convinced we are right, but that is what one thinks who is deceived anyway!"
All I could say was what I think is the ONLY answer for the issue. Be as incredibly honest as as we can and always, always, always pray that God correct us if we're wrong. Deceived people think they're absolutely right! And pray God make us more honest if we're not honest enough!