The IRS will almost always cash a check received with a mailed-in return before processing the return.
As stated elsewhere, if you got a check and along with it some complicated filings you had to do to account for how you would spend it, descriptions of those expenditures as you made them, etc, you would certainly cash the check before working on all that paperwork.
To the fact that the IRS has not made your return shown to you might not be toubliing.
1) Unless the IRS sends you a letter that they haven't received your return you are not in jeopardy
2) Even if they do, if your check was in an amount that included any items the IRS wouldn't know, such as charitable deductions or vehicle mileage or medical expenses, then it can be argued they must have received it to know the dollar amount owed.
3) Do you have proof of the mailing (certified mail and or return receipt)? -- Those would mean you have a perfect defense.
