You cannot know because the IRS does not acknowledge the receipt of mail. That is why when mailing anything, it should be mailed certified with return receipt or with another tracking service. That it the only proof of mailing that the IRS will accept and the only way to know if it was received by the IRS.
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**
