char-charset is a built-in function in `C source code'.

(char-charset CH &optional RESTRICTION)

Return the charset of highest priority that contains CH.
ASCII characters are an exception: for them, this function always
returns `ascii'.
If optional 2nd arg RESTRICTION is non-nil, it is a list of charsets
from which to find the charset. It may also be a coding system. In
that case, find the charset from what supported by that coding system.