How to set sampling frequency of an acceleration data set
If you don't have a uniform time period (or nearly so) between samples, then you don't have a sample rate, and the FFT result may well be garbage.

If you don't know ahead of the sampling the  Nyquist frequency for the signal, then the sampling might be already contaminated by aliasing.

If the samples are close enough together relative to the highest frequency in the signal's spectrum, then you may be able to interpolate a new set of evenly spaced samples, which will then have a known sample rate (based on the distance of the new even spacing).