If you just need to find the maximum frequency, you could calculate the fourier transform and find out the highest frequency containing a significative amount of power. How did you obtain your data? Using an accelerometer? If so you should know the sampling frequency (and you should be sure this sampling frequency satisfies at least the Nyquist Fs>2*Max_Frequency).
Your code should look like this
Fs = %your sampling frequency;
NFFT = 2^nextpow2(L); % Next power of 2 from length of y
Y = fft(y,NFFT)/L;
f = Fs/2*linspace(0,1,NFFT/2+1); %Single side spectrum just the positive part of the spectrum
