On a 70 Charger how can I tell which upper control arm is left (driver side) and which is right (pass side)?
My steering doesn't feel right and won't return to center after making a turn, so now I'm second guessing on whether I installed them correctly. They are the original factory arms.

On the arm is a formed in pad for the bump stop that goes to the front, over the bump stop.

I see it now. Looks like I have them in correctly. Thanks!

Return to center could be either an alignment or a steering gear box too tight issue.

Stumpy is correct, positive caster is what makes the steering wheel return to center IIRC.
Less caster makes the steering wheel easier to turn, but it doesn't return to center as easily with less.

manual or power steering ?
returning to center requires positive caster, regardless of the steering box used. [just like stumpy said]
visualize a shopping wheel cart's front wheels. most times, the "wobbly" ones you push have had the front steer wheels crashed into many objects, therefore making them out of kilter. in other words, the caster angle of the pivot point of the wheels has been bent/broken/repaired wrong, etc. causing the wheels to want to go in a direction other than straight ahead.
manual box vehicles need a zero to a negative couple of degrees caster to be able to comfortably turn at slow speeds, plus a garbage can lid sized steering wheel.
power box vehicles can use a bunch of positive caster [5-6 degrees or more, depending on the application and driver preference] to want to "stay straight at speed" and have the ability to want to return to center after a turn, as well as use a smaller, more comfortable feeling, diameter steering wheel, because of the hydraulic assist the pump provides.
the one downside of lots of positive caster, is the "jacking effect" it causes at extreme lock. depending on the camber curve, and the toe curve, this can cause some handling issues in autocross events, or just feel weird to the casual driver.

it's power steering. Everything was completely apart for years. It's back together now with a rebuilt steering box.
I'm hoping that it's a caster issue and that an alignment will take care of the problem. But the steering wheel just doesn't seem to turn as easily as I remember. I know the old power steering was numb and really easy to turn. You could blow on the steering wheel and make the car turn. Now it seems too tight. Don't know if that will get better with an alignment or if there is a problem with the rebuilt steering box. I loosened the cross shaft adjustment screw on the steering box a half turn to see if that would help, but nothing changed.

would it be possible the rebuilt box was modified internally to be something like a "stage 2" valving ? [decreased assist]

I doubt it. It was just a standard rebuilt box from the local auto parts store.

Stumpy,
Can you elaborate on the "steering too tight" issue? What could be the possible causes?

The worm gear in the box is crowned and the free play/clearance must be adjusted with the pitman arm off and the box centered in it's travel.
Due to the gear being crowned if it is adjusted off center it will bind as it passes the center. The further off center the adjustment was done the more it will bind. it is something that should be checked and adjusted during every alignment but rarely is. Especially so with all new components

Google "Steering box adjustment". You can tighten the preload on the worm gear and it will help. There's a lock nut holding down either an allen screw or slot screw on top of the steering box. Mark the orientation of the screw as it can turn when you loosen the lock nut. After loosening the lock nut, turn the screw counterclockwise about 1/8 but not more than 1/4 turn.It doesn't take much to change the resistance. Tighten the lock screw and test drive it. The steering should center itself when you slowly drive out of a turn.You can tighten or lossen this until you return to center with very little play.