find tris and non planar poygons

find_non_planar.py

# This example assumes we have a mesh object selected

import bpy
import bmesh
import mathutils
from mathutils import Vector

tessellate = mathutils.geometry.tessellate_polygon
normal_of = mathutils.geometry.normal


mesh = bpy.context.object.data

for f in mesh.polygons:
    if len(f.vertices) == 3:
        f.select = True
        continue
    
    # 4 or more vertices. let's check coplanarity by
    v = [mesh.vertices[i].co for i in f.vertices[:]]
    start_normal = Vector()
    for idx, pol in enumerate(tessellate([v])):
        vectors = [v[i] for i in pol]
        if idx == 0:
            start_normal = normal_of(vectors)
        else:
            if (start_normal-normal_of(vectors)).length > 0.002:
               f.select = True
               continue