No easy way without contacting the previous owner. Were there any mods to the car when you bought it, how many miles? Most dont replace the clutch unless something goes wrong with theirs; usually after either lots of miles or lots of time spent above 300whp/tq (or just abusing it/not knowing how to drive stick well.) If they werent making lots of power, they typically dont go heavy aftermarket.
If you cant tell from an A vs B driving your car vs a stock focus, i would say its stock. Usually the engagement points and window feel different with aftermarket. Otherwise, its one of those mods you cant see, not like an aftermarket intake where you can just pop the hood and see an entirely different filter