This was a question that I really tried to answer. How does one recognize a modernist heresy? I read Pius X encyclical on modernism but still found it difficult to apply it. It is now 100 years later and the error must be still the same. I work with someone who attends SSPX masses and the charge of Modernism is applied often to those not connected to the SSPX.
I am not sure where I heard it but someone gave a definition for Modernism that brought all things together for me.
Modernism is denial of the supernatural
Any view,ideology,practice etc. can be examined through that lens.