The steering arms casting numbers can be looked at. Not many people will bother to swap them on a clone and they are not common replacement wear items. Bulkhead markings can help, but you have to tear the interior out to see them.
Casting dates on master cylinders can be checked but not many original ones will have lasted this long so....
While there are LOTS things to check, if the original block numbers are gone and it has no real paperwork.....You CAN find stuff to rule it out, but you will never be 100% sure it is authentic.