I don't know if it's the sun or the moon in the sky.这句话（语法）对吗？

It's hard to tell if it's the sun or the moon in the sky.

你的说法也没有错误。

翻译的话可以说，不确定天上究竟是太阳还是月亮。

Because sometimes the sun and the moon can be seen at the same time.

wonder作动词，可以用I'm wondering... 或 I wonder...