IF "%Variable%"=="" ECHO Variable is NOT defined
This should help but this works, provided the value of Variable does not contain double quotes.
Or you may try. Both worked for me.
VERIFY OTHER 2>nul
SETLOCAL ENABLEEXTENSIONS
IF ERRORLEVEL 1 ECHO Unable to enable extensions
IF DEFINED MyVar (ECHO MyVar IS defined) ELSE (ECHO MyVar is NOT defined)
ENDLOCAL
source http://www.robvanderwoude.com/battech_defined.php