I want to confirm whether I submitted a tax return to IRS for the year 2018. How can I confirm this information?

You can check to see if the IRS has a transcript of a 2018 return from you.

You can get a free transcript from the IRS:   

https://www.irs.gov/individuals/get-transcript

You can also look in your online account--- if you have one-- to see what it says about your 2018 return.  If it shows a 2018 return, then you need to see whether it says it was "accepted," "rejected,", "printed," or "ready to mail."  If it says anything other than "accepted" you did not file it---unless you printed it, signed it, put it in an envelope and took it to a mailbox your self.

Do you know how to find your account?

Many people have multiple TT accounts and forget how to access them.  Log out of the account you are in now.

Account recovery

https://myturbotax.intuit.com/account-recovery/

https://ttlc.intuit.com/questions/1901535-forgot-your-turbotax-online-user-id-or-password

Or did you use the desktop version of TurboTax?  If so, the files are on your own hard drive or any backup device you used like a flash drive.

https://ttlc.intuit.com/questions/1901486-how-many-turbotax-accounts-do-i-have

https://ttlc.intuit.com/questions/1901535

https://ttlc.intuit.com/questions/1901659-find-your-tax-data-file-in-mac

https://ttlc.intuit.com/questions/1900721-find-your-tax-data-file-tax-file-in-windows

To get a copy of your previously filed returns prepared with online TurboTax https://ttlc.intuit.com/questions/1900748-how-do-i-get-a-copy-of-a-return-i-filed-in-turbotax-online

https://ttlc.intuit.com/questions/1900748-how-do-i-get-a-copy-of-a-return-i-filed-this-year-in-turbotax-online

Note:  If it turns out that you did not file your 2018 return, you can still do so, but it cannot be e-filed.  It has to be printed, signed and mailed.

https://ttlc.intuit.com/community/prior-year-return/help/how-do-i-finish-a-prior-year-return-that-i-started-in-turbotax-online/00/26322