You are correct! We do not know what is in the heart of man.
Maybe we should therefore take a slightly different approach when talking to others.
In our conversation with them, we should a) try to determine are they Christian or not. If we feel that they are not believers, then we can b) begin to explain what God has done for us and how He has changed our life. We can also c) invite them to our church to hear His word proclaimed. The invite may need to be repeated many times. So rather than saying "He died for you", perhaps it is better to say "only He can save you, which is true".
I think that item “c” above may be the most important. For, how can you believe what you have not heard? But since we are unable to change them, we must rely on God and realize that only He can change the heart. This could result in the person being called, but He could also use this to harden them!
It must be tough to be a “minister of the word” and realize that many are called but few will come! However, when success does come, what joy must fill their heart watching the change take place.