bytestring-encodings-0.2.0.2: checks to see if a given bytestring adheres to a certain encoding

isUtf8 firsts calls the very fast isAscii to see if the data is ASCII (and thus UTF-8). Use this if you know most of your data is ASCII-encoded. If you know that most of your data is probably not UTF8-encoded, it is probably best to use isUtf8' to avoid this check.
isUtf8' does not call isAscii. Use this if you know most of your data is not ASCII-encoded. If you know that most of your data is probably ASCII-encoded, it is probably best to use isUtf8.