According to TurboTax Easy Extension, you should have received an email after filing your extension request that would prove your acceptance. So go back through your emails around the date you filed your extension to find the email. Be sure to check your spam file, too.
Here's a link to a TT article that explains in more detail:
What to Do After You've Filed an Income Tax Extension - TurboTax Tax Tips & Videos (intuit.com)
*** I am NOT a tax expert. I am a seasoned TurboTax user, and volunteer to provide assistance to TT users. Nothing I post is to be considered TAX ADVICE; I bear no legal liability for responses.***
