Simple (?) Windows question

Is there some way that a CL implementation can know whether it is running under a Unix-alike like Cygwin or MinGW? If we use RUN-PROGRAM to interrogate the environment, we do so using CMD.exe, so I'm not sure how CMD.EXE would know if it was being run as the grandchild of a bash process. AFAICT Cygwin (and I don't know about MinGW) don't put anything into their environments that I can obviously see. E.g., if I start a lisp inside cygwin and do (GETENV "OS") I just see "Windows_NT". Thanks, r