In Odoo, each contact under a company (customer) is linked through the parent_id field on the res.partner model. Here’s how to identify that a contact belongs to a particular customer:

1. Parent Company Field: If you open the contact record (e.g., YYY Contact1) in form view, you'll see a field typically labeled as "Related Company" or "Parent Company." This field (parent_id) shows the company record (e.g., XXX Customer1) to which the contact is linked.
2. List View with Parent Column: To view this link in the list view, you may need to add the "Parent Company" column in your view. You can do this by customizing the view or adding a filter that shows the parent_id relation. This way, it will be clear in the list view that YYY Contact1 is a contact under XXX Customer1.
3. Advanced Search: You can also use Odoo’s advanced search feature. Go to the Contacts list view and apply a filter to show records where the parent_id is the specific company (e.g., XXX Customer1). This approach will list all contacts linked to that company.

These methods help visualize and verify the hierarchical relationship between a customer (company) and its contacts within Odoo.