God Himself must have given Joseph a docile Spirit to understand God's dealing with him right from the time the angel appeared to him telling him to take Mary as his wife and not to deal away with her for she had conceived by the power of the Holy Spirit I think from this moment Joseph knew the voice of the angel of the Lord and would distinguish it from indigestion.

The begining of every thing created both visible and invisible is God and therefore God was the initiator of our salvation hence He made Joseph to understand and take the responsibility of making sure that all was okey with the child Jesus till he was big enough to begin His ministry.