Frequency response of a filter - fft vs. freqz
If you have the filter coefficients, freqz is the way to go - the various optional input arguments and the expected outputs are documented in MATLAB which you can see using help freqz. Snippets given below:

" freqz Digital filter frequency response.
    [H,W] = freqz(B,A,N) returns the N-point complex frequency response
    vector H and the N-point frequency vector W in radians/sample of
    the filter:

[H,F] = freqz(B,A,N,Fs) and [H,F] = freqz(B,A,N,'whole',Fs) return 
    frequency vector F (in Hz), where Fs is the sampling frequency (in Hz).  "

If you have filter's impulse response - which will be different from the filter coefficients - you can use it to find the frequency response of the filter using the FFT.