How to calculate the frequency of a signal without knowing the sampling frequency using Matlab
If you do not have the information about the sampling frequency $F_s$ of your digital data, the best option is to talk about dimensionless relative frequencies $f$, or reduced frequency. The frequencies you observe on periodograms will be $f = F/F_s$, where $F$ would be the true frequency (hoping you have no aliasing). This amounts to saying that your sampling period is $1$ (dimensionless) and that the maximum observable frequency in your signal is $1/2$.

However, it is likely that the actual sampling frequency can be obtained from the data file, the  experiment or sensor, a recorded phenomenon with known frequency (like the 50 or 60 Hz power) or the person who gave you that signal.