I been sanding My car down to get it ready for some paint + for some new sheet metal to be welded in places... (its a project car). The stamped rear quarter on RH just before the door has been Touched up something shocking. There is Nicky (or however its spelt) about half an inch thick, the whole side there has been squashed in (didnt know).

Question:

How would I know if the car is bent out of shape... warped... twisted?

(hopefully its just the panel and the chasis is still perfect.)

One way would be to drive the car down a dirt or sandy road and see how the tire tracks look. That will tell you if the frame is bent from side to side. It's a poor boy way to diagnose a bent frame. The best way would be to put it on a frame machine and check it. Any frame or good body shop should be able to do this for ya. Other than that it's pretty much an eye ball measurement or try taking some measurements from front to rear and side to side at the axles. Good luck.  

It sounds to me like your car isn't driveable at this time, so here is what I would do. Get a service manual, and inside the service manual will be a frame dimension chart. Take the required measurements as described by the manual, and that will tell you if anyting is bent. If you are not comfortable with doing that, get a service manual, and take that and your car to a frame shop and have them check it out for you.