As others have said, ...there is no perfect way to confirm a '68 big block, ...however (as has been said) the car had to be an SS to get the big block, ...so confirmation of the SS items (ie. 12-bolt; raised hood; signs of original emblems; 5 leaf springs; etc. would be a start.
The items for a big block that would differ from a small block...: the heater core, fan shroud, fuel line, engine frame mounts, etc.
If the drivetrain components are in question, ...check the dates compared to the cowl tag/car's build date; single leaf rear would be 6 cyl or 327 (ie. non SS).
Tim trax, ...if as you say the motor is non original, ...and a 10-bolt rear, ...have you checked the 3rd digit of your VIN#? (ie. 3 or 4?). If that checks (ie. it a 4, 8-cyl.), ...I'd then check the date on the rear, ...if the rear date is close to the build, ...then it's probably not an SS...