How can I differentiate between a Multivalued Attribute and an Entity? While designing ER Diagrams, how can we differentiate between a Multivalued Attribute and an Entity?

How can we know that, what we are considering as an 'Entity', isn't actually an entity but a Multivalued attribute or vice-versa?
Ask yourself if the thing under consideration continues to have an existence if all the other things in your model go away. If so it's an entity, if not it's an attribute.  Does the thing you are considering have a way to distinguish it from all other items in its class i.e. a primary key? Then it's an entity.

The answer you come to may depend on the context in which you are working. For one computer application a particular value may be treated as an attribute. In another it may be an entity. This is not a contradiction; it is a natural outcome of simplifying the real world into a model and in doing so selecting the important parts and excluding the others.

If you intend to implement your model you should recall that first normal form requires that each attribute hold only one value.