i recently took a CPR course and we discussed seizures quite a bit. Basically, try to protect them but don't fuck with their mouths (don't put anything in there, even a benzo), don't roll them to their side, just let them trash around and call 911 (or whatever number is applicable to your area for ER services). Try to make sure they don't smash their heads off the ground when they fall and that's about all you can do.
There are signs that can happen up to a day before the seizure occurs, some common symtpoms are confusion, light headedness, extreme anxiety, shaking definitely, muscle twitches and things like that. I swear i was a few minutes from having a seizure once during a phenazepam withdrawal but went home took it easy and things got a bit better and i didn't seize. I have no idea why i've never had a seizure even from withdrawing CT from huge doses of benzos but it still fucking scares me.
source: my cpr instructor (they could be wrong, but i've felt the "warning: here comes a seizure" feeling before and what she described as the symptoms preceding a seizure were exactly what i felt.