Post-ictal seizure question. How do you know what happened if alone?

You may not. Postictal state is the abnormal condition between after a seizure and before returning to your baseline condition. Impairments may still be present during this state similar to having a seizure. Bumps, bruises, lacerations may be the only documentation of an event if you are home alone. Bathroom, kitchen and general home safety should be scrutinized if you remain alone and independent.
By circumstances., If alone, with no one to tell you what happened, your would only be able to know what happened by the circumstances in which you find yourself. You might have bitten your tongue, or your cheek, lost bladder or bowel continance or having fallen.