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.
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.