Hello there, AB111.

If the invoice reminder has been sent, the status is displayed in the invoice's activity as You: Sent this Invoice regardless if this is only a reminder or an actual invoice.

You'll want to specify that email sent is a reminder, you can enter an indication in the Activities field. This way, you'll know if the email sent is just a reminder or an actual invoice. This section also lets you see if the email is viewed by your customer.

Please see the screenshot I've provided below: 

Please let us know if you have further questions. We'd be around anytime.