Hi,
The invoice should show a "Partially Paid" or "Not Paid" status after the credit note is applied, and you should also see a different balance on the invoice, IF the credit note was applied to this invoice, specifically, and not on the customer account.
When creating a credit note, at the end of the process select "Retain on customer account" so you can then apply it to one particular invoice. Open the invoices list, select the invoice, click Credit Note from the top menu, select the customer and credit note and apply accordingly.
Regards,