Java BASE64 utf8 string decoding

I'm using org.apache.commons.codec.binary.Base64 do decode string which is utf8. Sometimes I get base64 encoded string which after decode looks like for example ^@k��@@. How can I check if base64 is correct or if decoded utf8 string is valid utf8 string?

To clarify. I'm using

public static String base64Decode(String str) {
    try {
        return new String(base64Decode(str.getBytes(Constants.UTF_8)), Constants.UTF_8);
    } catch (UnsupportedEncodingException e) {
         ...
    }
}

public static byte[] base64Decode(byte[] byteArray) {
    return Base64.decodeBase64(byteArray);
}