You know that they got it when they cash the check - *if* they cash the check.
If lost in the mail (or lost at the IRS which happens quite often) and the check is never cashed, then without a post office tracking there is no other proof that the IRS will accept that it was ever sent at all.
**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.**
