Are there even Christian Japanese bands out there?
If so, how would you know if they are really singing Christian music?

???? Learn Japanese ????

I know of a couple. One is Nobuaki Irie. He sings in both English and Japanese. Another is a duet called Afterglow. They only have one album in Japanese that I am aware of. It is called "Warriors of Light" and the songs are the Japanese versions of their English songs on other albums.