As much as something can be said to exist it can be said to posses form.
Form may be determined by the internal component structures of a body, or by other factors such as the internal component structures of other forms interacting with the primary form. Thus internal structures or processes of some bodies which exist in the environment of a specific body may be said to subjectively determine the form of the body in question, though of course this does not necessarily change the formal aspect of the body in question.
Between observers, attributes, or even the existence of a particular form may be disputable. This, however, often has more to say about a particular observer than about the phenomena (body, primary form) in question.
A form interacts with its environment. This is one way in which its existence can be described, or "proven".
The absence of an observer's belief in an existent form may in fact negate the existence of the form for that observer. The effect of this is more of less negligible depending on the strength of the form's interactions within the shared environment between the form and the (non) observer. This shared environment may be described in terms of an array of vectors of interaction which describe strength and proximity in the relation to and object existent and one perceived.
Of course, this is how a person may be struck by a car they didn't realize existed.