Identify which program created the .DAT file
Open it with the original program if possible
Try a text editor such as Notepad or TextEdit
Try a hex editor if the file is not readable as text
Use a media player if the .DAT file contains audio or video data
Use a spreadsheet or database program if the file contains tabular data
Rename the file extension only if you know the correct file type
Check the file source or related software documentation
Use an online file identifier tool to detect the file type
Scan the file for malware before opening it