To view which reminders have already been sent to a client, hover your mouse over the quote expiration date.

This will show you the timeline of which reminders have been sent and which reminders are upcoming.

If you're looking to add more reminders or adjust the reminder schedule, you can "Click to Edit"

You'll see the date the quote was sent as well as which reminders have already been sent/are upcoming. At any point you can have up to three reminders pre-set - if two reminders have already been sent, you can add two additional reminders, for a total of five reminders to be sent to your client.