They just shake their limbs back and forth or their body. It's pretty easy for anyone familiar with seizures to tell a fake, since most fakers shake their limbs back and forth evenly - a sort of even sawtooth pattern of movement. However, jerky movement in seizures is contraction-relax-contraction-relax, contracting in one direction only, not back-and-forth jerking. People also aren't conscious during full-body seizures (though they are if it's partial-body), so if they're looking around focusing on things, looking at your response, or talking, that's a giveaway.

One point to note, though, is that you can't just assume that everyone faking a seizure doesn't have epilepsy. Quite a lot of fakers do, and they fake for different reasons. A lot like the attention they get after a seizure. Others come in to have an EEG and think that they need to have a seizure to help the diagnosis, so they 'help out' with good intentions (not common, but common enough). Very very rarely you might encounter a real seizure that looks like a fake, though these are generally other kinds of seizures (various kinds of 'partial' seizures), not the full-body-shaking ones.