1.16.5 How to check if entity is leashed

You can use #isLeashed, but it's not for all the entities. It's rather for some of its subclasses. For example, Horse#isLeashed is a method. Meanwhile, Player#isLeashed is not.

 

You will have to listen to the leash and unleash event and do a list, where you store all entitys in and than check if the entity is in the list

EDIT: As ZBLL said, I tested and the isLeashed() method is in LivingEntity Class, so everything that is implemented from a LivingEntity you can check if it is leashed

 

LivingEntity has a method is Leashed

 

Other entities can‘t be leashed anyways.

Just do this:

Code (Java):

public boolean isLeashed(Entity entity) {
  if(entity instanceof LivingEntity) return ((LivingEntity) entity).isLeashed();
  return false;
}