how can we tell that OTSU binarization is feasible or not.
Assuming you have MATLAB, you can just look at the source code, where you will see that the EM is defined as


  em = maxval/(sum(p.*((1:num_bins).^2)') - mu_t^2);


where

I = im2uint8(I(:)); # the input image
num_bins = 256;
counts = imhist(I,num_bins);
p = counts / sum(counts);
omega = cumsum(p)
mu = cumsum(p .* (1:num_bins)');
mu_t = mu(end);
sigma_b_squared = (mu_t * omega - mu).^2 ./ (omega .* (1 - omega));
maxval = max(sigma_b_squared);