Welcome to the Community - thank you for your question!
Is this an invoice you have emailed?
Sorry for the delay, the question applies to estimates (normally the first occasion to contact the customer) and invoices.
You can view the status a few ways.
1.Click on sales>all sales>make sure you have the status column ticked on(in the cog above the word action)>use the filter to filter to specific customers/dates etc
2.Click into the invoice>scroll down>it should say viewed if it has been viewed
3.click into the invoice>click more>click audit history>click the date and it will have a set viewed status
Any questions let us know