There should be a physical copy book structure for the way in which the data to be stored into the segment. You have to use that same copy book to look the data into the segment.
I hope use cant have any copybook for the whole DB.
One more way to find the data in the unload PS is, find out one value of a field in a particular segmnet. From there onwards you have start your own research with the available copybook structure of that segment.
As you said it is a FLAT file it should have a copy book .
Your shop may have any tool to find out that .
There may be chance to unload only one segment or more then one.
If it is one segment means in ps it is single layout structure.
If it is more then one segment means it is multi layout structure(XREF).
XREF dataset you have to ask who knows well in your shop.
You can use that and browse over the data .