When you start your world, Minecraft will function normally but as you progress, strange things will start to happen. The first symptom will be when your heart bar starts to change, you will have 4 hearts, then 37, then 10 hearts and this will not stop.
You must also be careful with the water in the villages because at some point they will turn into lava and the roof of the buildings will start to burn.
Then as night falls, you will get messages warning you that you should not be in this place, telling you to leave the game.
After the messages you will know that the end is near as the screen will begin to flicker and it will become night for a moment and then the sun will rise again, all in the space of a few seconds.