It is easy to determine the default charset when it isn't marked because it is specified in the standards. For MIME, and text content types, the defualt charset is US-ASCII (RFC 2046). For HTTP, the default is ISO-8859-1 (RFC 2616).

However, most browsers will interpret the charset based on local settings. I can set Firefox to use any encoding as the default and read unmarked Shift_JIS files if I want.