Just print your tax return. How you do that depends on weather you used the online version or the desktop version of the program. is definitive proof, you can get a transcript directly from the IRS at https://www.irs.gov/Individuals/Get-Transcript
The Electronic Postmark report will be included in the PDF of your tax return if you download the PDF to include All forms and schedules.
If you have filed your 2014 tax returns and they have been accepted, when you sign onto your online account and are on theWelcome Home screen, click on Taxes at the top of the screen. When you are on your 2014 tax timeline, under the section for SOME THINGS YOU CAN DO, click on Download all forms and worksheets
Once the PDF has been downloaded to your computer, you can view or print the return with any PDF compatible viewer, such as Adobe Reader.
Or you can go to the following TurboTax website for the status of the e-filed tax return. If the tax return was received and accepted, there will be date/time stamps for when it was received and for when it was accepted. Click on the Print this pageicon in the upper right corner of the screen - https://shop.turbotax.intuit.com/efile/efile_status_lookup.jsp