I guess what you're saying is that you could have a file with no idea what data it contains? If the file has no extension or identifying information within the file, then you can try importing it into something like Audacity to see if it's raw audio data. There are also "graphic explorer" utilities where you can examine the data visually to see if there's any bitmap data. There's also a chance the data could be compressed, so only the program it was created with could make sense of it. A hexademical file viewer (e.g. Hex Workshop) might be useful.
For printed info, if there's not too many digits, you could use Hex Workshop to create the actual file and then analyse it.