How do I check if my client has viewed the estimate/invoice I’ve sent him?

You would receive a notification in the bell icon on the top right corner stating that your customer has viewed the invoice/estimate.

Also you can simply click on the particular estimate or invoice and check if it has been viewed by the client. A tiny eye icon on the right corner suggests that the client has seen it.

Alternatively, go to the estimate or invoice tab on the home page of Zoho Invoice. Click on the drop down next to All estimates or All Invoices and select Client viewed.

You can now see only the client viewed estimates/invoices.