Can someone give me some tips on communications (digital)

I have a file containing sample data, now I have done the FFT on that
data and plotted it out.

looks rather like this
|
|  /
| /
|/
/
_____

The point of the exercise is to determine the bandwidth of the sensor
that this file came from. I am not sure as to where to go from here.
I am studing via correspondence and the set text doesn't go into this
into detail.
I am thinking it is more than just reading off the graph??

Thanks Mick Carey