crutledge,
Assuming you are on Windows, you can use the batch file (epubcheck_1.1.zip) you can find here: (
https://www.mobileread.com/forums/sho...d.php?t=201764).
You'll also need this one:
https://code.google.com/p/epubcheck/...bcheck-3.0.zip
Alongside FlightCrew, it gives you another way to check your epub with an easy drag and drop.