If this life is a dream, then how to know the real self?
You cannot know the real self. What you can know is that everything is changing.
Once you know everything is changing, one who feels that everything is changing is non changing. Without the non changing, you cannot recognize the changes.
Without a reference point, how can you say something is changing? That reference point is not the object of knowing but it is the knower himself.
If you want to know everything is an object of knowing, then it is not possible. Don’t make the ultimate an object of knowing.