If you enter a character that’s incompatible with the document’s current encoding, it may appear garbled when you reopen the document. The Warnings inspector lists characters that can’t be represented in the current encoding, making it easy to check whether the document contains any incompatible characters.
This feature is also useful when converting between encodings with different character repertoires—for example, from Japanese (Mac OS) to Japanese (Windows, DOS), or from Unicode to a local encoding.
In the CotEditor app 
on your Mac, open a document.
Choose View > Inspector > Warnings.
To close the list, do one of the following:
Choose View > Inspector > Hide Inspector.
Drag the inspector’s separator to the edge of the window.