If you filed you extension before today you should get an email notification within 48 hours, however it could take longer. You can also go to the TurboTax Check
Your E-File Status website
for the status of the e-filed extension. If the extension was received and
accepted, there will be date/time stamps when it was received and
accepted.
*If you just filed your extension today:
"The IRS has announced that they are having technical difficulties accepting e-filed returns (and extensions). This impacts all tax preparers and all tax returns. Taxpayers should go ahead and continue to prepare and file their taxes and extensions as normal with TurboTax.
TurboTax has uninterrupted service and is available and accepting e-filed returns. We will hold returns (and extensions) until the IRS is ready to begin accepting them again."