He is adopted....right?
OK, here is the difference, the Spitfire gets the cover of Vogue and the P40 gets the cover of New Jersey Monthly.
The Spitfire can be handled by Englishmen while it , with rare exception, takes a Colonial to handle a real airplane such as the P40.
Spitfires suck for nose art application whereas the P40 was designed with noseart areas in the specs.
Spitfires seem to fu....errr mate like bunnies if the numbers of Spitfires encountered in online sims is to be believed.
Spitfire landing gear do not do that cool turn backwards thing when raised.
There are probably a lot more ways to tell them apart but above is a start.