I've compared the $_SERVER superglobal in both invocations. It seems that $_SERVER['argc'] (i.e. number of arguments passed to the script) is only set when running from shell/command line:
That seems to work both on Linux and Windows hosts. (First I thought about checking for some of the environment variables, but those are different for every operating system. Also, all the $_SERVER['HTTP_*'] headers are missing in the CLI version, but I'm not sure if that's reliable enough.)
