“And Zacharias said unto the angel, Whereby shall I know this? for I am an old man, and my wife well stricken in years.” (Luke 1:18 KJV)
A preshared key. One of the most common means of authentication between two established points such that they can communicate confidentially and with some assurance that the message has not been tampered with. Even if that message is transmitted out in a very public forum. Time after time in the Bible, we see God establishing a clear thing that He intends to do, and above and beyond the questions of ‘doubt verses faith’, we see how God (or a direct messenger from God) delivers this announcement. He gives a key that perfectly descrambles the situation after it has been fully delivered. We notate what has been said. The events are often outside of what we can manage by ourselves. Often outside what we can obtain help for. Thereby allowing us to see what He has done and know with an incredible certainty that His hand was indeed there.
If that was not enough, He also has given us a means to communicate His presence, power, compassion, and realness between any two of those that are His. A means that the world has no real understanding of until they see Jesus first.
“By this shall all men know that ye are my disciples, if ye have love one to another.” (John 13:35 KJV)