Identify the file source and type
Try opening it with a text editor
Use the program that created the DAT file
Open it with a media player if it contains audio or video data
Open it with a spreadsheet or database tool if it contains tabular data
Use a hex editor if the file is binary
Rename the file extension only if you know the correct format
Use file conversion tools if needed
Check the file properties or metadata for clues
Search for the associated application from the file’s folder or source