Verse: Luke 1: 18 : Zechariah said to the angel: How shall I know this? For I am old, and my wife is advanced in life. "
"How shall I know this?" The question is reasonable. How can one have a child in the old age? . But the part of the question "how do I know this is ", was not expected from him who was a priest. Because he knows that Isaac, Samuel etc had baby when they were old by the blessing of the God. So asking for symbol was not allowed. St. Mary asked angel " How can this be? ". The angel explained it.
But question by Zechariah has distrust in there. He is asking for proof. So angel told him. ` The proof is since you disbelieved my words which will be fulfilled in their time, you will be dump."
Zechariahs looked at the circumstances first, and what God can do last; we are tempted to think this is logical; but if God is real, there is nothing logical about putting circumstances before God
Verse Luke 1:19 " I am Gabriel who stands in the presence of God". Gabriel means.( angel of the Power of God) (Daniel 8-16, 9:21) and Michael means "Who is like God? "(Daniel 10:18, 21,12:1,Joshua 5:9, Rev 12:7). Apart from these two names third one "Rafael is also mentioned in Tobit 12:15). Gabriel reminds Zechariah where he is from and what good news that he has brought for him and for the people of Israel.
( More about Angle Gabriel http://www.newadvent.org/cathen/06330a.htm
( About angels in general http://www.newadvent.org/cathen/01476d.htm