You might have - if your address is, say,
try sending an email to:
If whoever's providing your email service is using a fairly standard flavour of mailserver software, then it should end up in the mailbox for email@example.com.
Basically, the plus sign and everything between it and the @ is ignored when working out which mailbox to put the email into at your ISP's mailserver.
If it does get through (as opposed to bouncing 'user unknown') then the "To:" line of the email you get will have the firstname.lastname@example.org address in it, which you can then filter by.
NB: This all works fine until your mail provider changes to use software that doesn't operate in this way, at which point all the addresses with plus signs in them that you've invented over the years will bounce - so this approach isn't perfect. Useful in certain situations though.