finding frequency range and dominant frequency of a stored signal
To detect the fundamental frequency of your signals, you can use self-correlation. The maximum of the self-correlation will correspond to your fundamental frequency (the second maximum, as self-correlation is always maximum at 0). You can then interpret the lag value of this secondary maximum to find the periodicity of your signal, and the amplitude of the maximum to evaluate the strength of periodicity.
note : The value of the self-correlation at lag=0 is the energy of your signal. This can help as a scale for the periodicity strength.

Otherwise, to visually analyse the spectral content of your signal, you can just apply a Fourier Transform to it and plot the magnitude of the Fourier Transform.