How can I find out the dates that I filed my previous years taxes

[DoninGA]

Your prior year tax returns will have a Postmark Report if you e-filed the tax returns.  The Postmark Report will have the date/time the tax return was e-filed and the date it was accepted.

To access your current or prior year online tax returns sign onto the TurboTax website with the userID you used to create the account - https://myturbotax.intuit.com/
Scroll down to the bottom of the screen and on the section Your tax returns & documents click on Show. Click on the Year and Click on Download/print return (PDF)

[damang8]

Where in the return will the postmark date show?

I efiled the 2018 return but the date in the signature field is blank.

[xmasbaby0]

The date you e-filed is not going to be on your Form 1040.  It is going to be on the sheet you print from your pdf showing you filed electronically.