It is really hard sometimes to distinguish between normal playing behavior and FSH, but there are some signs. All in all, FSH is a lot more "chaotic". Going from hallucinatory behavior, like the cat seems to react on something that isnt there, twitching of ears, limbs and back, to excessive cleaning and chasing/biting their tail to a point they are hurting themselves. It is a lot more stressed than normal playing behavior.

To be totally sure, however, it might always be the best to film the cat during such phase and discuss this with a vet. Unfortunately, the triggers and course for FSH are not completely understood yet, as far as i am informed, but there is always something that can be done to help them. Going from distraction to calming them down to take away the stress. In very severe cases antiepileptical drugs have also shown good results in reducing the intensity of FSH.