Two things - send yourself a BCC to one of your email addresses. That the only sure way of knowing it was sent.
And, even though it is not proof that the email went out, you could script in a routine to add a new record in a related table to track all emails sent to each individual. That related record could contain date or timestamp, and other particulars including the text of the email.
Some of the plug-ins can provide additional details which may help you.