There is no way to tell for certain, but if there is an earth soldered onto the lead sheath of the incommer then chances are pretty high that its always been TN-S
Also the age of the earth clamp will give you a fair idea. The old two part cast iron clamp, and old style BS951 clamps are a good indicator.
People were not as concerned with earthing back in the day, so were less likely to try and connect it them selves.
Also the area will play a part. Round here it is very very rare to see a subterranian TT supply.
In the local area where I live, I can't remember working on a TT supply ever, but 5 or so miles away that is all you will find.
With my last high Ze, the DNO tried it on on the phone. I'm not sure if it's a deliberate ploy, or just the woman in the call centre who didn't really understand what I was telling her.
She kept saying that is is the customers responsibility to provide an earth, but they could provide a PME terminal for £75.
I eventually got through to her that it was a failed existing TN-S connection, and after 15 mins on the phone, they sent the emergency chap to have a look.
He put his tester onto the MET, and said "it's fine mate, look, I've got 0.34Ω"
I asked him to disconnect the bonding and try it again
Anyway, next morning they turned up, swapped the 30A cast iron cutout for a nice new BS1361 100A head and provided a PME terminal, all free of charge