Ngon filter?

Is there an ngon or triangle filter to show clearly where the are in the mesh?

There are a bunch of addons. I use this one.

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OTOH, to answer the question itself:

  • in Face selection mode, deselect everything
  • do a Select -> Select All by Trait -> Faces by Sides
  • in operator options, set Number of Vertices to 4 and type to “Not Equal To”

This will select all non-quads in your mesh.