Put some tie-wraps tightly around the shaft of the rear shocks and push them up against the body of the shock, then go for a drive over some bumps. If the tie-wraps are now down at the bottom of the shaft, your shocks are bottoming out. If not, look for shiny spots under the car around suspected contact points. You can put duct tape in those areas to see if you are indeed getting contact.
Lowering the car while keeping the suspension from bottoming out requires higher spring rates, which will result in a harsher ride. You may not be able to have both a good stance and a good ride. Do you know what the spring rate is for your new springs (e.g. 225lbs/in)?
