If the image is always cropped and normalized like that you can just check for black pixels at a few locations in each square. Otherwise, you could try Template Matching to find the sudoku grid and crop out the inner squares, again checking for black pixels.
If the image is always cropped and normalized like that you can just check for black pixels at a few locations in each square. Otherwise, you could try Template Matching to find the sudoku grid and crop out the inner squares, again checking for black pixels.