You can square up a wall before raising it by measuring from opposite corners. Assuming that both the top and bottom plates are equal in length, and that both ends are the same, the wall is square when the diagonal measurements are equal. Adjust the wall as necessary, then keep it square with sheathing or with diagonal braces before raising it.
Before you do any deep digging or drilling on your property, be sure there are no underground utilities where you plan to work. Call DIG SAFE at 1-888-DIG-SAFE (1-888-344-7233).