java CharsetDetector: How to detect ASCII

i am using CharsetDetector to detect the charset of a text file.

This is the code to detect the charset of the given file:

private String getCharset(File file) {        String charset = "";        try {            InputStream is = new FileInputStream(file);            BufferedInputStream bis = new BufferedInputStream(is);            CharsetDetector cd = new CharsetDetector();            cd.setText(bis);            CharsetMatch cm = cd.detect();            if (cm != null) {               Reader reader = cm.getReader();               charset = cm.getName();            }            bis.close();            is.close();        } catch (IOException e) {            // TODO Auto-generated catch block            e.printStackTrace();        }        return charset;    } 

For a ASCII text file it returns UTF-8. ASCII is a subset of UTF-8 but i like to detect ASCII if it is ascii only and UTF-8 if there is a sign which is not in ASCII.

But how can i check it?