First, if it has the original drivetrain, you can decode those to see if they match the VIN of the car. An original big block will surely be an SS. If no X code is present on the cowl tag, it could be an early built car or it was built in LA. Some other things to look for are - welded steel brace on drivers side rear frame to mount a tail pipe hanger, front power disk brakes, etc.. I couldn't put a value on anything I can't confirm.
You can do a search in the Tag Team section to find out more info on decoding.
'69 Z11 Pace Car, 350/4 speed, manual top, console w/gauges, tilt rosewood wheel, AM radio