I would probably look for an abundance of control characters which would typically be present in a binary file but rarely in an text file. Binary files tend to use 0 enough that just testing for many 0 bytes would probably be sufficient to catch most files. If you care about localization you'd need to test multi-byte patterns as well.
如前所述，您總是很不幸，得到一個看起來像文本的二進制文件，反之亦然.
As stated though, you can always be unlucky and get a binary file that looks like text or vice versa.
