If you mean spec as in SE, Sport etc, then there's no real defining edges - they blur as original owners added and modified items from the options list.
By and large, cars with 15" wheels are base spec, 16" wheels are SE and 17" are Sports, but there's other tell tale signs too: front fogs and 4 spoke steering wheels point to SE spec. 3 spoke steering wheels, Sport seats and full driver info displays point to Sport models, but there are crossovers.
Cheers,
Mike