in my experience when in a domestic house when the supply cable is inside the house directly near the fusebox it normally is a tns system and its easy to see off the sheath of the supply cable in to earth block etc.A house i went to exactly the example i mentioned it had an old earth wire looked to be part of the sheath but i could not tell it came out at the bottom of the supply cut out old style cut out bs 88 fuse style cut out i presume the earth can be tagged on actually inside the cut out. Im guessing its a tns if tncs it would be going in to side of the neutraul block these systems are outside normally in a electric box the supply cut out id say tns has to be