Also, if I want to check the frequency of a waveform what would be an accurate way to do the same? Should I use the

fft

command from ngspice or is there a better way to do the same?

I use fft, plot in xschem and also octave. See below thread for a good discussion on ffts in ngspice: https://sourceforge.net/p/ngspice/discussion/127605/thread/03b82f8bf2/