How do I know if I suffered a seizure when I recently passed out?
It sounds like you suffered an attack of vasogagal syncope, which is a largely benign overreaction of the nervous system to certain stimuli. Ie your IBS symptoms resulted in your fainting episode.
Seizures are generally determined when witnessed, although certain telltale signs (such as loss of urinary or bowel continence) might suggest a possible seizure. There are, however, many types of Seizures and it is difficult to comment on without further information.
If this occurs frequently it might be best to be examined properly by a medical physician. In any case, the head injury you sustained might warrant a trip to the doctor for a quick assessment as it is.
I hope this helps, but as mentioned, do get a quick check to ensure you're alright.