How can I check if a directory exists?

You can use opendir() and check if ENOENT == errno on failure:

#include <dirent.h>
#include <errno.h>

DIR* dir = opendir("mydir");
if (dir) {
    /* Directory exists. */
    closedir(dir);
} else if (ENOENT == errno) {
    /* Directory does not exist. */
} else {
    /* opendir() failed for some other reason. */
}

Use the following code to check if a folder exists. It works on both Windows & Linux platforms.

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

int main(int argc, char* argv[])
{
    const char* folder;
    //folder = "C:\\Users\\SaMaN\\Desktop\\Ppln";
    folder = "/tmp";
    struct stat sb;

    if (stat(folder, &sb) == 0 && S_ISDIR(sb.st_mode)) {
        printf("YES\n");
    } else {
        printf("NO\n");
    }
}

You might use stat() and pass it the address of a struct stat, then check its member st_mode for having S_IFDIR set.

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>

...

char d[] = "mydir";

struct stat s = {0};

if (!stat(d, &s))
  printf("'%s' is %sa directory.\n", d, (s.st_mode & S_IFDIR)  : "" ? "not ");
  // (s.st_mode & S_IFDIR) can be replaced with S_ISDIR(s.st_mode)
else
  perror("stat()");