<p>How do you identify the file content as being in ASCII or binary using C++?</p>

Well, this depends on your definition of ASCII. You can either check for values with ASCII code <128 or for some charset you define (e.g. 'a'-'z','A'-'Z','0'-'9'...) and treat the file as binary if it contains some other characters.
You could also check for regular linebreaks (0x10 or 0x13,0x10) to detect text files.