How to Determine Structure of a Data File

I am trying to use a DAT file from an old sleep monitor (ZEO). I have downloaded a library and sketch examples that were written for this file type several years ago. I am able to get some of the examples to work with my file, but would like to modify and extend them. My question is, how can I determine the variables and functions that are available in the DAT file?

Hard to say

Is it a binary file

Did you try to open it with text editor