How do I check if a string is unicode or ascii?

What do I have to do in Python to figure out which encoding a string has?

In Python 3, all strings are sequences of Unicode characters. There is a bytes type that holds raw bytes.

In Python 2, a string may be of type str or of type unicode. You can tell which using code something like this:

def whatisthis(s):
    if isinstance(s, str):
        print "ordinary string"
    elif isinstance(s, unicode):
        print "unicode string"
    else:
        print "not a string"

This does not distinguish "Unicode or ASCII"; it only distinguishes Python types. A Unicode string may consist of purely characters in the ASCII range, and a bytestring may contain ASCII, encoded Unicode, or even non-textual data.

How to tell if an object is a unicode string or a byte string

You can use type or isinstance.

In Python 2:

>>> type(u'abc')  # Python 2 unicode string literal
<type 'unicode'>
>>> type('abc')   # Python 2 byte string literal
<type 'str'>

In Python 2, str is just a sequence of bytes. Python doesn't know what its encoding is. The unicode type is the safer way to store text. If you want to understand this more, I recommend http://farmdev.com/talks/unicode/.

In Python 3:

>>> type('abc')   # Python 3 unicode string literal
<class 'str'>
>>> type(b'abc')  # Python 3 byte string literal
<class 'bytes'>

In Python 3, str is like Python 2's unicode, and is used to store text. What was called str in Python 2 is called bytes in Python 3.

How to tell if a byte string is valid utf-8 or ascii

You can call decode. If it raises a UnicodeDecodeError exception, it wasn't valid.

>>> u_umlaut = b'\xc3\x9c'   # UTF-8 representation of the letter 'Ü'
>>> u_umlaut.decode('utf-8')
u'\xdc'
>>> u_umlaut.decode('ascii')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 0: ordinal not in range(128)