How to select the sampling frequency when the input signal frequency is not known
When the input signal bandwidth in unknown, you can begin with the highest possible sampling frequency, then observe the energy in the high frequency region (close to Nyquist frequency; i.e. half the sampling rate) and try to estimate the bandwidth of the signal and then choose the lowest possible rate still maintainig the Nyquist criterion according to this accepted bandwidth.

If the first trial indicates that there's considerable energy towards the Nyquist frequency, then you should try more complex sampling strategies, such as bandpass sampling at higher frequency regions, unless you can afford an even higher frequency sampling device.