$ cat checkpw.c
#include <sys/types.h>
#include <stdio.h>
#include <unistd.h>
#include <limits.h>
#include <userpw.h>

int main(int argc, char *argv[])
{
        char *username, *pw, *msg = (char *)NULL;
        struct userpw *up;
        int rc, reenter;

        if (argc > 2) {
                fprintf(stderr, "Usage: %s [username]\n", argv[0]);
                return 2;
        }

        if (argc == 2) {
                /* username is in argv[1] */
                username = (char *)calloc(strlen(argv[1])+1, 1);
                if (!username) {
                        fprintf(stderr, "can't allocate memory for username\n");
                        return 3;
                }
                strncpy(username, argv[1], strlen(argv[1]));
        }

        if (argc == 1) {
                /* get current user */
                username = (char *)calloc(PW_NAMELEN + 1, 1);
                if (!username) {
                        fprintf(stderr, "can't allocate memory for username\n");
                        return 3;
                }
                rc = getlogin_r(username, PW_NAMELEN);
                if (rc) {
                        fprintf(stderr, "can't get current user's name, RC=%d\n", rc);
                        return 4;
                }
        }

        printf("Checking password for user %s\n", username);

        pw = getpass("Enter password: ");
        if (!pw) {
                fprintf(stderr, "can't obtain password for user %s\n", username);
                return 5;
        }

        do {
                rc = authenticate(username, pw, &reenter, &msg);
                if (msg) {
                        if (!reenter) {
                                fputs(msg, stderr);
                        }
                        free(msg);
                        msg = (char *)NULL;
                }
        } while (reenter);
        return rc;
}

$ cc checkpw.c -o checkpw

$ grep -p user /etc/security/passwd
user:
        password = {ssha512}06$verylongpassword
        lastupdate = 1448619478

$ ./checkpw user
Checking password for user user
Enter password:
$ echo $?
0