If I can't remember when I sent my taxes how to check

The pdf with your return would have a page of electronic filing instructions if you e-filed.  If you mailed your return, only you would know when you took the envelope to a mailbox.

And if you have not deleted your emails from last year--check your emails.

When you e-file your federal return you will receive two emails from TurboTax.  The first one will say that your return was submitted.  The second email will tell you if your federal return was accepted or rejected.  If you e-filed a state return, there will be a third email to tell you if the state accepted or rejected your state return.

You can check your emails and look for an email from Intuit TurboTax or you can sign into your online account (assuming you used TurboTax Online) and click on Tax Home in the left panel and then scroll to the bottom of that page to see the details of your previously filed tax returns.