How can I check if an invoice has been read by the client after emailing,

You can check the invoice Status, Steven.

You'll want to go to the Invoices list to see if your client read it. Once your client receives the invoice email and clicks the Review and pay button, this will mark the invoice status as Viewed, which means the client reads it. Let me show you how:

1. Go to the Get Paid & Pay menu and then select Invoices.
2. Find the invoice in question.
3. From the Status column, make sure it changed to Viewed.
   

In addition, you can click on the invoice to see how often your client views it. Please read this guide for more status insights: Create invoices in QuickBooks Online.

Moreover, I like to add this page to learn more about recording and processing different kinds of payment transactions and also to help your client with their needs: Manage payments.

As always, I'll guide you if you need more QuickBooks assistance. Anytime I can help. Stay safe.