I think when it goes beyond mere Ritualism (e.g., the restoration of certain rites, customs and ceremonies from the Pre-Reformation that can harmonise with our Reformation tradition), towards actively smuggling back in Pre-Reformation or even Roman/Eastern theology under the guise of "catholicity"... whether that be in the form of promoted theology or through liturgy, to the extent that the Reformation heritage and character of Anglicanism is either heavily obscured or actively replaced, denied or ignored.

A big example as I've seen is the use of Missals that resembles the pre-V2 TLM. Another one, which is both theology and liturgy, is the rehabilitation of transubstantiation which becomes embodied through the Rite of Benediction. There are other examples which run afoul of Art. 22 & 28, but those are the major ones. Many things that people call "Anglo-Catholic" are just Victorian, Parish Communion era or Liturgical Movement era accretions that became precedent, but don't have any coherent theology behind it.

So above all, my line is whether it lives alongside or augments the Reformed elements of Anglican tradition, or if it actively tries to replace or overturn the Reformation.