Apply your filter to an impulse and look at the respnse.
Assuming that your filter's spatial size is no greater than 2*d in either dimensions, you can create a delta-function image like this:
testim = zeros(2*d+1, 2*d+1);
testim(d+1, d+1) = 1;
Then apply the filter to testim as you would to any other image, and the output is the impulse response. You can use FFT2 on it if you want the result in the frequency domain.
