How did Mary know she was pregnant with Jesus?

Jesus kicked
[filler characters]

she was told by an angel.....a messenger....

Presumably by noticing that her period had stopped, followed by cravings for whatever ancient Israel had in the way of ice cream and pickles.

She was likely between the ages of 11 and 15 .... 
and assuming that she did not have any weight issues 
or hormonal problems, she probably ceased menstruating 
while she was pregnant. 

Told by angel Gabriel.  Being from the priestly line, she knew prophecy. 

Probably because she started missing her period. People were stupid 2,000 years ago, but not that stupid. I'm sure she also knew that a spirit didn't make her pregnant.

Yeah, we don't anything in the way of every day pregnancy stuff from the Bible, let alone Mary's birthing experience. She just serves as a literal vessel with no other function and no more humanity than a bowl. She has no personality, nothing is recorded about her other than her usefulness as an incubator. We don't get to hear that she had problems or if she suffered during this pregnancy. Or who helped her, what other women were there. Did she give birth among strangers? How horrifying that would be. We don't get a retelling of what herbs or oils were used...Mary is just a throwaway character in an old story that centers on men and the doings of men and gods...with no thought to women at all, other than as vessels. 

Just like all other women, I would presume. Scripture does not develop Mary's character enough to infer much from it.

If you bothered to read the account, you wouldn't ask that stupid question.

Oh dear, it seems you're too young to be on this site.

Are you kidding? 

I bet giving birth while still a virgin would hurt double bad. She wasn’t even loosened up yet I bet. Ouch!

According to this photograph, a little bird told her!

https://rlv.zcache.com/virgin_mary_with_doves_lily_roses_classic_round_sticker-r11c5656ec7e346ee813cbdd7969f375c_0ugmp_8byvr_307.jpg

When she took the pregnancy test, the rabbit died.

Fortunately, he rose from the dead on the next spring equinox, which is why we have Easter.

The angel Gabriel said that she would become pregnant without having sexual relations with a man. But Gabriel did not offer to explain to her family and neighbors how she would become pregnant. What would they think? How would she convince Joseph that she had not been unfaithful to him? In addition, she had the great responsibility of raising God’s Son as a human! We do not know all of Mary’s concerns, but we do know what she did after Gabriel spoke to her. Please read Luke 1:38 And Mary said, Behold the handmaid of the Lord; be it unto me according to thy word. And the angel departed from her.

Luke 1:31 brings out "And look! you will become pregnant and give birth to a son. and you are to name him Jesus. it was thru an angel of God 

Luke1:1-33
The angel Gabriel told her

   God sent an angel named Gabriel to announce her pregnancy. He informed her, she had found favor in God’s eyes. God’s Holy Spirit would come over her and she would be pregnant with God’s son and to name him Jesus. This is recorded in the Bible at Luke chapter 1 verses 26-38.

From the revelation of Gabriel described in Luke 1.

Same way other women know they're pregnant.

An angel appeared and told her.  She was confused initially. 

Why don't you just read the Bible and find out for yourself?  GOD told her. Now read the Bible and get details! 

Well, she would have know Father God.

It never happened, none of them ever existed.