When someone dies you often hear "He/She is in a better place.". HOW DO THEY KNOW THAT?

They don't know that.. is the short answer.. The longer answer is... they believe it, or they want to think you believe it.. It's an attempt to comfort and console... generally it's said with good intentions... but the truth is no one knows.. 

There's some evidence to suggest that having faith of a religious nature can help ease the grieving process.

Some do, some don't. 

They get an anonymous text from Claire Voyant ....  Maybe....:( 

They don't.  It's something people say to give comfort to those "left behind".

They are full of it. Unless the newly deceased was enduring a life of unrelenting misery or utter hopelessness, I would call BS on such a disingenuous platitude. 

By any other comparison, I can’t help but think that life in any measure is an infinitely better place than being dead.
Ask yourself, was it a better place for you before you were born? 

They are in a better place.  Don't you know we all go to Dave and Busters after we die?

Well, they either say it because they want to believe there is a meaning to life and a better place than this exists, or they think that not suffering anymore is a better place i.e the same better place we speak about when we put pets "to sleep."

It is a hopeful feeling toward someone who has passed.