Identify the program that created the DAT file
Open the DAT file with that same program
Try a text editor such as Notepad or TextEdit
Try a hex editor for binary DAT files
Rename the file extension if you know the expected format
Open it with a media player if it contains audio or video data
Use a file converter if the DAT file is from a specific application
Check the file properties or source location for clues
Search online for the app associated with the DAT file
Use a file viewer tool that supports DAT files