JSON's default encoding is UTF-8:
http://www.ietf.org/rfc/rfc4627.txt
From section 3:
JSON text SHALL be encoded in Unicode. The default encoding is UTF-8.
Since the first two characters of a JSON text will always be ASCII characters [RFC0020], it is possible to determine whether an octet stream is UTF-8, UTF-16 (BE or LE), or UTF-32 (BE or LE) by looking at the pattern of nulls in the first four octets.
This determination is unambiguous so there is no special place where an encoding is described in the format itself.