You need to request a receipt, which you can do when you compose the email (options/return receipt). But as OG says most people configure their email clients to ignore such requests, so while receiving one will indicate that the email has been opened, not receiving one indicates nothing at all. You can configure how your own copy of TB responds (or not) to such requests by going to Tools/Options/Advanced Return Receipts. By default this is (I believe) set to "never send a return receipt".