How to test the FIR filter in C++? I am almost done with design and implementation of my first FIR filter using C++. Its a Sinc-Windowed FIR filter designed for a cut off frequency of 100 Hz with sampling rate of 500, with a filter order M = 100. All the operations have been carried out in discrete time domain. So basically my question is regarding testing to see if my filter is generating the desired response . 

I presume one way to test the output would be to apply DFT to it and check the response spectrum of the filter in the frequency domain, but for this I have to develop a function that performs the DFT on the output .

Question :To avoid this, I am looking,

---->Are there any methods that make it possible to test the filter response in the (without the need for applying DFT) time domain itself?.  

----> If the above is not possible, are there are any alternative methods like some C++ DSP library with FT function's you can suggest? 

Or Any suggestions and advice regarding this with your experience would be appreciated. Thanks in advance!
The first basic test could be to use a unit impulse as an input signal and see if the output signal equals the impulse response (i.e. the filter coefficients). Another simple test signal is a unit step. The corresponding output should be the cumulative sum of the filter's impulse response, i.e. for $x[n]=u[n]$, the output must be

$$y[n]=\sum_{k=0}^nh[k],\quad n\ge 0$$

If you obtain the desired response for these two simple test signals, that's a good start, but I wouldn't stop there.
In general it is very helpful to use a tool like Matlab or Octave (free), or anything similar, which has built-in filtering routines against which you can check your routine for arbitrary input signals.