I'm new to blender. When whole mesh's shading is weird. I can notice that. but how you determine small part's shading good or bad?
I suppose you're asking if there's a way to see if the normals are all facing the right way? Try to be as specific as you can when posting questions.
You can check this in the viewport overlay dropdown menu, under geometry check the "face orientation" checkbox:
Red means it facing the "wrong" way.
to fix the normals so they face the same direction:
- select object
- 'TAB' to enter edit mode
- 'A' to select everything
- 'ALT+N' to get the normals menu dropdown
- select Recalculate Outside