SRFI 170 says:

#o140000   local domain socket

But this is for all sockets - not just local-domain ones but internet,
etc. This prints "socket":

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#include <sys/types.h>

#include <sys/socket.h>
#include <sys/stat.h>

#include <arpa/inet.h>
#include <netinet/in.h>

#include <stdio.h>
#include <unistd.h>

#define PORT 23456

int main(void) {
   struct sockaddr_in addr;
   struct stat st;
   int fd, is_socket;

   if ((fd = socket(PF_INET, SOCK_STREAM, 0)) == -1)
     return 1;
   addr.sin_family = AF_INET;
   addr.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
   addr.sin_port = htons(PORT);
   if (bind(fd, (struct sockaddr *)&addr, sizeof(addr)) == -1)
     return 1;
   if (listen(fd, 10) == -1)
     return 1;
   if (fstat(fd, &st) == -1)
     return 1;
   is_socket = (st.st_mode & S_IFMT) == S_IFSOCK;
   printf("%s\n", is_socket ? "socket" : "not a socket");
   if (close(fd) == -1)
     return 1;
   return 0;
}
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