C++11 - Unicode string - find if contains non-ascii based characters

I have Unicode string in C++11 represented in UTF8 class. I can get Unicode character numbers. How can I check, if the string contains characters, that are not ASCII based (with diacritics)?

Eg. I want to detect Japanese, Arabic, Russian etc., but for special characters like German ü, Czech č, French î etc. I want to said, that it is "ASCII like"

(I dont want to use Boost)