You say you aren't sure if you checked the box. The box is on the ADS form, if you filed electronically you would have uploaded that form as a PDF. You should be able to look at the PDF and see if you checked that box or not. If you did not check it there is no way to change it for this application.
According to the USPTO's MPEP 1122 Requests for Nonpublication
A nonpublication request must be filed upon the filing of the application. < This is a statutory requirement and cannot be waived. > For example, a nonpublication request filed with a request under 37 CFR 1.53(c)(3) to convert a provisional application to a nonprovisional application will not be accepted as timely filed because the nonprovisional application would be accorded the original filing date of the provisional application if the request to convert is granted. The nonpublication request must also be included with the application papers. The nonpublication request cannot be filed separately on the same date as the filing date of the application(e.g., the nonpublication request is filed in a different “Express Mail” package than the package that contains the application). < If the Office mistakenly accepts an improper nonpublication request, applicants should notify the Pre-Grant Publication Division and rescind the request immediately. See MPEP § 1730 for contact information.
If it were critical, one could file the whole thing all over again - with the box checked - today. You would get a new application number and the filing date would be the same. After you are sure every thing was correct in the second filing, the first application could expressly abandoned. Of course you would be paying the fees all over again.
All of this is tricky and please don't act solely on this educational information.