They have released an update that now allows you to check your world seed! If you want to share a world with people, you can now see which random seed you were given and give it to people. For instance, the seeds for the worlds that I play on are:
5946633467530347600
2154920
888
-140572620216272678
The way to look up your seed is to go to select Play Game, select the world you want to look up the seed for, and right under the world name it will say which seed was used to create your world. The seed won't appear unless you save the world with the latest update.
Also, as I've said before, if you use a seed that isn't preprogrammed into the game (such as your gamertag, name, etc.) then it will select a random seed for you to start on. This is a way to check if your seed is an actual seed or not.