First of all check the body panels and items you can see under the bonnet, doing this before you go to the hassle of getting alignment checked can save you money as you might find something that makes the car beyond economical repair.
- Check the roof skin/panel, if its simply dented down due to the ground hitting it it can be replaced, but if it's warped due to the pillars moving in this could get quite costly or non-economical.
- Check the windscreen, as above if it's cracked due to the ground then replace but if its cracked because the pillars have moved its not impossible but can be costly to repair.
- Check the door shut gaps. If the gaps have got significantly larger or smaller then this indicates that the body has distorted, again not impossible to fix but will require straightening on a jig. Also check
- If the vehicle has mechanical gear linkage check all the gears are present and correct, this is often overlooked but can indicate the transmission has shifted or the gear linkage has been hit.
- Check the boot/tail gate gaps, if one side is large and one small this could just mean the hinges have moved but if one side is much larger than the other this could mean the rear end has shifted across, this isn't that rare and can be hard to see but it usually can be fixed on a jig.
- Check the engine, if the car spent a long time on its roof or side oil could have found its way into the cylinder and will hydraulic lock the engine, if the engine runs all good, if not just remove the spark/glow plugs and turn the engine over for a few seconds, if the engine smokes run it for a while as its probably just oil burning off.
- Check the coolant level, it may have just leaked through the cap if the spring is weak or its an older vehicle, so top up run the engine and check for leaks.
- Check the physical location of the engine/transmission make sure everything is correct and no engine mounts have broke.
Obviously if all of the above is damaged then it is probably beyond economical repair but if theres only one or two and the car is of high value then move onto checking the chassis and underside alignment.
You will require specialist equipment for the following:
Check the tracking, camber and caster of all the wheels. If only a few degrees then this can probably be adjusted out but it would be worth checking why its out, such as moved mounts, steering rack, or bent arms.
Cross measure as many points underneath as possible, you can do this manually and make sure its within a couple mm, a better way is with a digital measuring system which uses a computer and known values to cross reference.
Check for crushed or moved chassis rails, sills and panels etc.
None of this is an exact science, it takes years of experience to look and inspect a vehicle to see if its worth repairing.
For example it would be nothing to replace both side panels, roof skin and windscreen after a roll over on a 2 year old Merc, Range Rover etc. But you wouldn't entertain it if it was a Peugeot or Renault or an older vehicle.