If you need to test for the existance of an environment variable, you might consider using:

if not "%var%"

"" @echo var=%var%

While this works, a better method is:

if defined var @echo var=%var%

Try this little test:

@echo off
if not "%var%""" @echo line 3 %var%
if defined var @echo line 4 %var%
set var=hello world
if not "%var%"=="" @echo line 6 %var%
if defined var @echo line 7 %var%