extern "C" int cygwin_posix_path_list_p(path); 
const char *path;

This function tells you if the supplied path is a POSIX-style path (i.e. posix names, forward slashes, colon delimiters) or a Win32-style path (drive letters, reverse slashes, semicolon delimiters. The return value is true if the path is a POSIX path. Note that "_p" means "predicate", a lisp term meaning that the function tells you something about the parameter.

Rather than use a mode to say what the "proper" path list format is, we allow any, and give apps the tools they need to convert between the two. If a ';' is present in the path list it's a Win32 path list. Otherwise, if the first path begins with [letter]: (in which case it can be the only element since if it wasn't a ';' would be present) it's a Win32 path list. Otherwise, it's a POSIX path list.