Existing Wording
A client can determine that an order has not been performed by searching for order resources with no matching responses (see below)

Proposed Wording
A client can determine that an order has not been performed by using the Order id to perform a search for Order Response resources that contain a Reference to this Order: if none are found, then there is no record that the order was record (it is possible that the order was performed but the act has not yet been entered into the system).

Comments
First, "see below" does not appear to have text below that explains it.
Second and more significant: "searching order resources with no matching responses" does not appear possible based on the Order search parameters. What field can be queried to indicate the existence or absence of responses? There is no response status or reference. My proposed text may resolve the problem, though I'm not sure if that is the correct way to do the query, and it seems like it wouldn't work if the Order id is unvalued (which it can be, given cardinality of 0..1).
Also I added the parenthetical remark because I think it's wise to be cautious about assuming negation (absence of performance) if that has not been asserted.

Grahame's Comments
agree