It can be difficult to tell the difference unless you speak Japanese or Chinese. The poem could be a Chinese classical poem written by a Japanese person. The poem could by a Japanese poem written in Chinese style (kanbun). For this poem, I took one of the phrases and put it into a search engine, and that led me to discover the poem and the poet. But until I did that I wasn't completely sure what or where this came from. Well, I had an idea that it isn't Japanese because some of the kanji used are not found in Japanese - but even still I wasn't 100% sure.
It's not uncommon to write a classical Chinese poem in a Japanese scroll, so "a real Chinese poem is used there" doesn't prove that it's a Chinese scroll. As for your scroll, as Majestic-san mentioned, 兰 is a simplified Chinese character mostly used in Mainland China or Singapore, and never used in Japanese. Similarly, 异 is used as an alternative of 異 there, but this usage is found only in modern simplified Chinese. 异 is also used in Japanese, but it can't mean 異.