or are you trying to find out if the string contains any non-ASCII characters, ie characters that need specific character maps or multiple bytes to encode?
eg "Nineteen characters" will generally encode to 19 bytes but "15 characters
" will be longer than 15 bytes because the "smiling face with halo" character is Chr(128519) which is going to need at least three bytes to represent it, because a single byte can only hold 256 different values, usually 0..255 for oldtimers, or -128 to 127 for newbies.