$ iconv -f UTF-8 your_file -o /dev/null

>>> a="γεια"
>>> a
'xcexb3xcexb5xcexb9xcexb1'
>>> b='xcexb3xcexb5xcexb9xffxb1' # 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

>>> try: print b.decode("utf_8")
... except UnicodeDecodeError, exc: pass
...
>>> exc
UnicodeDecodeError('utf8', 'xcexb3xcexb5xcexb9xffxb1', 6, 7, 'unexpected code byte')
>>> exc.args
('utf8', 'xcexb3xcexb5xcexb9xffxb1', 6, 7, 'unexpected code byte')

$ apt-get install moreutils
$ isutf8 your_file

///Returns -1 if string is valid. Invalid character is put to ch.
int getInvalidUtf8SymbolPosition(const unsigned char *input, unsigned char &ch) {
  int                 nb, na;
  const unsigned char *c = input;

  for (c = input;  *c;  c += (nb + 1)) {
    if (!(*c & 0x80))
        nb = 0;
    else if ((*c & 0xc0) == 0x80)
    {
        ch = *c;
        return (int)c - (int)input;
    }
    else if ((*c & 0xe0) == 0xc0)
        nb = 1;
    else if ((*c & 0xf0) == 0xe0)
        nb = 2;
    else if ((*c & 0xf8) == 0xf0)
        nb = 3;
    else if ((*c & 0xfc) == 0xf8)
        nb = 4;
    else if ((*c & 0xfe) == 0xfc)
        nb = 5;
    na = nb;
    while (na-- > 0)
      if ((*(c + nb) & 0xc0) != 0x80)
      {
          ch = *(c + nb);
          return (int)(c + nb) - (int)input;
      }
  }

  return -1;
}