Check if an environment variable is defined without command extensions and without using a batch file?

Question

I need to use a cmd.exe command line (cmd.exe is being called from the gyp build tool) to determine whether an environment variable is defined or not. How can I do this? I

Answer 1

Errr... just:

if defined your-var-name ( 
    echo yarp
) else (
    echo narp
)

I should add, I do not believe this needs command extensions...