Plotting the frequency response of a FIR filter
If you want to do it manually, it is not upsampled by adding zeroes between coefficients. It is usually done by adding extra zeroes after the real coefficients. When padded to some convenient power of two like 512 coefficients, FFT can be used. The resulting amplitudes need to be scaled due to the padding. Matlab and Octave have freqz function to plot this directly for you.