Is there any way to know the quality of the carbon frame? I mean not the knockoffs. Even the major brands, even those brands that appear in the Grand Tours.

What I see is the price of frames with the major manufacturers. A company such as Specialized has different price levels, as do many other brands. The guesswork is to look at what are being used by the teams in the Grand Tours. Even then, the manufacturers may have done something special for those teams that they support.