The function tokenize.detect_encoding() detects the encoding either in the coding cookie or in the BOM.  If no encoding is found, it returns 'utf-8':

When result is 'utf-8', there's no (easy) way to know if the encoding was really detected in the file, or if it falls back to the default value.

Cases (with utf-8):

 - UTF-8 BOM found, returns ('utf-8-sig', [])
 - cookie on 1st line, returns ('utf-8', [line1])
 - cookie on 2nd line, returns ('utf-8', [line1, line2])
 - no cookie found, returns ('utf-8', [line1, line2])


The proposal is to allow to call the function with a different default value (None or ''), in order to know if the encoding is really detected.

For example, this function could be used by the Tools/scripts/findnocoding.py script.

Patch attached.