A good way of finding out translations are covers with Chinese, Japanese, or Korean language. 90% chance it is a tranlsation.
Another way is if pronouns aren't consistent, or the way the story is told seems weird. The way other languages tell their stories tend to be REALLY different then English. Its especially noticeable when machine translated to English.
Even novels written in English, inspired or based on the writing styles of other languages will still look and be structured mostly like English, so it will be really easy to tell.
Any translation that is like, properly translated probably wouldn't be here either, instead being on like, a proper translation dedicated site, and aggregators for sure aren't going to proof read 300 chapters when they can just plop a machine translation down and be done with it, so you can even easier tell. AND languages that aren't English have like, 20 different ways of saying the same pronoun, which machines get confused by really easy, and just put "He" down every time
so if there's an akwardly large amount of male pronouns, you can tell then too