Currently, there's no option to access your client's invoice portal to see what they see on the portal.
However, I've got an alternative method you can perform so you're able to view what they see from the invoice you sent to your client. I suggest adding your email address when you send an invoice to your client.
Click the Plus icon at the upper right corner.
Select the customer and fill in the other nessesary fields.
Go to the Customer email option and click the Cc/Bcc link.
Enter your email in the Cc field. Then, click the Done button.
Hit Save and send.
You can also ask a screenshot from your client on what they see from the invoice you sent to them.
Please check this article to see the different ways on how you can send invoices to your customers in QuickBooks Online: Send an invoice.