If you have already found your customer, then you can either select the patient from the Customers|Information screen, or you can click Patients in the top menu and use the Select button (useful if the customer is a boarding kennel and has 300 patients recorded).

If you are looking for a patient and have not already found the customer, then Patients|Information|Select may be used.

As shown below, if you omit any customer information, but provide some or all of the patient name, you will be shown all the matching patients, irrespective of owner.