There are two ways to see which Customers have been emailed reminders.

Within the Customer menu, display the table of Customer names:

Like so:

Click on a Customer and then History:

From the screen above you can also re-send any emails.

Or you could find this Under Admin Menu/Setup:

Click on EMAILS then Email History:

From this table you can view Email History against a Customer, sort by columns and so on.

You can also view previous emails and re-send: