How to detect watermarking in DWT~ Include matlab source~ please help me~~

how to detect watermarking?? I don't know how about detecting watermarking code please help me! I successed insert watermarking but I can't exctract watermark

clear all; clc % read gray image gray_img = imread('body_1_gray256.jpg'); % convert double image gray_img = double(gray_img); init = 0.0001; %

% [p q] = size(gray_img);

% sign_img = imread('sign_image.jpg');

[Lo_D,Hi_D,Lo_R,Hi_R] = wfilters('haar');

[cA,cH,cV,cD] = dwt2(gray_img,Lo_D,Hi_D);

% watermarking start y = [cA cH;cV cD]; Y = y + init*abs(y).* double(sign_img); p=p/2;q=q/2; for i=1:p for j=1:q

        ncA(i,j) = Y(i,j);      
        ncV(i,j) = Y(i+p,j);   
        ncH(i,j) = Y(i,j+q);   
        ncD(i,j) = Y(i+p,j+q);  
    end
end

Watermarked_img = idwt2(ncA,ncH,ncV,ncD,Lo_R,Hi_R);