How to check whether a file is valid UTF-8?

You can use isutf8 from the moreutils collection.

$ apt-get install moreutils
$ isutf8 your_file

In a shell script, use the --quiet switch and check the exit status, which is zero for files that are valid utf-8.

You can use GNU iconv:

$ iconv -f UTF-8 your_file -o /dev/null; echo $?

Or with older versions of iconv, such as on macOS:

$ iconv -f UTF-8 your_file > /dev/null; echo $?

The command will return 0 if the file could be converted successfully, and 1 if not. Additionally, it will print out the byte offset where the invalid byte sequence occurred.

Edit: The output encoding doesn't have to be specified, it will be assumed to be UTF-8.

Use python and str.encode|decode functions.

>>> a="γεια"
>>> a
'\xce\xb3\xce\xb5\xce\xb9\xce\xb1'
>>> b='\xce\xb3\xce\xb5\xce\xb9\xff\xb1' # note second-to-last char changed
>>> print b.decode("utf_8")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python2.5/encodings/utf_8.py", line 16, in decode
    return codecs.utf_8_decode(input, errors, True)
UnicodeDecodeError: 'utf8' codec can't decode byte 0xff in position 6: unexpected code byte

The exception thrown has the info requested in its .args property.

>>> try: print b.decode("utf_8")
... except UnicodeDecodeError, exc: pass
...
>>> exc
UnicodeDecodeError('utf8', '\xce\xb3\xce\xb5\xce\xb9\xff\xb1', 6, 7, 'unexpected code byte')
>>> exc.args
('utf8', '\xce\xb3\xce\xb5\xce\xb9\xff\xb1', 6, 7, 'unexpected code byte')