It is too late to file an extension, an extension had to be filed by July 15th.  If you are due a refund there is no penalty for filing a late return.  If you owe taxes, a penalty & interest accumulate until the taxes are paid in full.

You can also check your e-file status, it will show accepted, rejected, or no return filed.

How do I check my e-file status?

https://ttlc.intuit.com/community/return-status/help/how-do-i-check-my-e-file-status/00/25538