Something to note - it's not great to call it a "fake" seizure. I think the preferred term is psychogenic vs neurogenic. Psychogenic seizures are still a medical problem. They just have different origin. That's still someone who needs help. I think it's still helpful to be able to spot the difference though, because it can help the patient get the right kind of treatment for whatever is wrong. It can also be risky to label something a psychogenic seizure if you aren't completely sure, so be very cautious with your suspicions. General rules of thumb:

patients can't talk to you during a grand mal seizure -I've had patients talk to me while thrashing on the floor and that's something I'm pretty comfortable labeling psychogenic.

Patients typically are postictal for at least a few minutes after a grand mal seizure. If someone comes out of a seizure and is immediately AOx4, that raises my index of suspicion. Some patients do have short postictal periods after neurogenic seizures though so this is not 100%.

Typically patients are incontinent during a grand mal seizure. Not 100%.

Typically your heart rate is elevated right after a seizure. If their heart rate is normal, again this raises index of suspicion.

Hopefully this was helpful.