Data Inspector should reveal invisible (non-printable) characters in any String and Workbench should warn the presence of such characters.
For instance: from x{0000} to x{001f}, x{007F}, x{0081}, x{008D}, x{008F}, x{0090}, x{009D}, x{00A0}, x{00AD}.
How should a human being tell the characters above and a blank space apart if there is no distinction between them in FME User Interface? (I did to copy and paste inside Notepad++ to figure this out).

