Bottom line: there’s really no way to tell from just the filename.
The problem is that many applications use the file extension “.dat” to indicate a file that contains data.
But that’s exactly all it tells us. It doesn’t tell us what kind of data or how that data is formatted and represented.
In other words, it doesn’t give us a single clue as to what the file is or what program we should use to open it.
We need more information and there may be one way to get some.