how do I calculate SNR of noisy signal?
The common definition of SNR is the power of the wanted signal divided by the noise power. Suppose you have obtained the wanted and the noise signal as arrays, calculation of the SNR in Matlab before noise reduction can be done like this:

snr_before = mean( signal .^ 2 ) / mean( noise .^ 2 );
snr_before_db = 10 * log10( snr_before ) % in dB


After noise reduction, the residual noise can be calculated as the difference of the wanted signal and the actual signal. Calculation of SNR is then straightforward:

residual_noise = signal - noise_reduced_signal; 
snr_after = mean( signal .^ 2 ) / mean( residual_noise .^ 2 ); 
snr_after_db = 10 * log10( snr_after )