[JessicaJohansson]

How to check quality of content ?

[Alexa]

Have to be an educated person.

[MSL]

 You should know well the language of the content and you should have enough knowledge about its topic. For example, if you don't understand Chuvash language and the content is in Chuvash, you can't say is it a quality content or just some joke. Or, imagine, it's in Hindi (or in Tamil, in Urdu... and so on, which is your native language in India) and you understand it but its topic is about geodesy, and you majored in sociology, so you have "little to no" (a.k.a "little to none") knowledge about the geodetic problems.
 Speaking SEO wise, as you probably know, it's to have (be) uniqueness, usefulness, correct grammar and spelling, reliable sources, user-friendly, helpful, interesting, trustworthiness, good on-page and off-page SEO, etc.
 There is also another point (another view), that the quality content Is defined by data, it achieves marketing objectives, ranks well in Google, has very good CTR and lots of social media engagement, also that it has higher conversion rates and does well on every channel, which is more subjective (I mean that it's related to the subjects (humans); the users; the groups of people) and it's more success oriented. (To read more about this point of view, visit Larry Kim's article https://www.wordstream.com/blog/ws/2017/03/15/quality-content-defined.)
 Some quality content isn't successful and vice versa.

[Dimitroff]

Knowing now. Knowing is good. I know.