Is this like 'God only knows' in English which we use when God is the only one who could explain smth
In Vietnamese and Chinese, the literal meaning is "the Sky knows"
"The heavens know" is more accurate.
Not more accurate. "Trời" literally and typically refers to the sky, unless used with other context such as "Trời biết".
A longer version is "Có trời mới biết". This means "Only God knows".