Though you can't know 100% sure what "they" consider quality you can make a very educated guess.

First of all, Quality is relative and you can almost say that what is quality depends completely on the keyword(s). You can't say: "This is a quality website." All you can say is: "This is a quality website for this keyword phrase."

But you can say some things about "quality". Look at the historical data that Google uses... Monitoring history implies monitoring activity. This activity is compared to average activity for all selected sites for a keyword phrase. If you´re more active than the average, your quality is higher, if you´re less active than the average, your quality is lower.

There are so many sites that just static on-page factors are useless now-a-days and as Google determined a couple of years ago, static off-page factors are also pretty much useless.

In my opinion, a huge factor in quality is how active a site is. If it doesn't change enough compared to the average, it will never rank high.

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