The easiest way is to use ptype with gdb.

Example:

foo.c

#include <sys/types.h>
struct st { int i; char c; };
typedef int Array[10];
typedef struct st St;
int main(void)
{ 
  size_t s1;
  ssize_t s2;
  pid_t pid;
  Array arr;
  St st1;
  return 0;
}

$ gcc -Wall -g -o foo foo.c
$ gdb foo
(gdb) b main
(gdb) run
(gdb) ptype s1
type = long unsigned int
(gdb) ptype size_t
type = long unsigned int
(gdb) ptype s2
type = long int
(gdb) ptype ssize_t 
type = long int
(gdb) ptype pid
type = int
(gdb) ptype pid_t
type = int
(gdb) ptype arr
type = int [10]
(gdb) ptype Array 
type = int [10]
(gdb) ptype st1
type = struct st {
    int i;
    char c;
}
(gdb) ptype St
type = struct st {
    int i;
    char c;
}

Like the above example,

ptype <expression or type name>

shows real type.

If the type is an array, it also shows its size. If the type is a struct, it also shows the type of the members of the struct. It's convenient.

You can probably find the real type using gcc -E option and looking into preprecessor's output like below, but it's bothersome and possibly needs hard work.

$ gcc -E foo.c | grep ssize_t
typedef long int __ssize_t;
typedef __ssize_t ssize_t;
(ommitted below)

In the above example (x86_64 GNU/Linux), you can find that ssize_t is
__ssize_t => long int

$ gcc -E foo.c | grep ssize_t
typedef long __darwin_ssize_t;
(ommitted)
typedef __darwin_ssize_t ssize_t;
(ommitted)

In the above example (Mac OS X), you can find that ssize_t is
ssize_t => __darwin_ssize_t => long