Half a century and I'm still finding out what it's like during a seizure. I'm on "auto-pilot" for anything from a few minutes to hours. During this time I'll carry on as normal, you'll notice nothing. Then comes the fit/seizure as is often shown on TV, that's when it becomes visible(and most of the damage done, either from hitting something or the fit itself). There's then a confused recovery, often looking up at the roof, ambulance or A&E.
There's different types, the one triggered by flashing lights is photosensitive epilepsy. You might recall the Japanese cartoon which triggered dozens of cases of that type over there including lots of kids who weren't classed as 'epileptic. The truth is we're all actually potentially epileptic but thresholds and triggers vary. The docs had no idea what triggered mine and fortunately my threshold seems to have increased. As to warning, I'm not sure but Ive had just two occurrences where I felt like I was going to black out in the last 15 years (room spinning, then darkness) and by immediately crouching into a ball and closing my eyes touch wood I've recovered. Although I've had similar occurrences also nothing been as dramatic. Events seem to have been triggered by standing stationary for a long time in heat. When I've actually fitted I haven't had any warning though and I've no idea what it feels like though I just kept on waking up in strange places with no recollection of how I got there :-/
