Character encoding in rich text

I’ve got an app using the C API to parse rich text fields(composite data). I am hung up on how to figure out

what character encoding method is used for text. In most cases the encoding is LMBCS, but I’ve seen examples where the encoding is codepage 437 (OEM). Is there a field in one of the composite data types that has this information? I have not been able to find any documentation on this, nor do I see any other message property that would have this.