As a final check when taping drywall joints, use a side light to check for smoothness. The light can be a halogen worklight, a troublelight, a lamp without a shade, even a larger flashlight. Hold the light close to the wall next to the joints in several places to reveal high and low spots, or bumps and depressions, that otherwise would be easily missed.
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.
When lifting a wall section into position, the bottom of the wall can slip across the floor deck, and even fall off. If you are short on help, a solution is to position the section as it lays flat, then drive duplex nails (with two heads) through the bottom plate into the floor. The nails will bend as you raise the wall to a vertical position. After securing, pull out the duplex nails.