“If I know someone who has developed the powers to see these worlds then I can believe it.”
No. you may believe them just on the surface. you will never believe in them thoroughly unless, you experience and see things by your own self.
And besides 31 realms theory, you will find much more theory that exist in Buddhism which is beyond this mundane lives such as ‘Abhiññāṇa'(psychic powers).
If ones can practice meditation and developed certain level of concentration, they also can practice to developed those psychic powers like divine eye, divine ear, or even the ability to read other’s mind.
Do you believe it..?
I even can tell you, there is, Sotapanna, Sakadagami, Anagami, that still exist in this era and teachings Dhamma.
But, can you believe this..?