Most breeders cut the tails off of longies, so unless you’re a breeder, you probably won’t encounter a longie.
Males might be somewhat larger than females.
Manx cats have broad faces.
White Manxes might have a softer coat than Manxes of other colors.
Eye color in Manxes varies. Green, hazel, gold, copper, and blue are all common. Some Manxes are odd-eyed (that is, each eye has a different color).
Manxes feel surprisingly heavy when lifted.
If you do have a Manx, you can provide it with a cat tree or cat house so it can have many opportunities to jump and play.
If you observe these symptoms in your cat, contact your vet for a diagnosis.