I would think that would not be too easy. The diameter of the L & N conductors in 2.5 mm² T+E is about 1.78 mm, so a fair bit of judgement would be required to use it as a guide when trying to distinguish between 2.1 mm and 2.5 mm (diameter) plugs/sockets.Or try baring some 2.5mm twin and earth cable as a measurement gauge.
Kin Regards, John