Finding the adjusted gross income for the prior year, when you filed jointly with your soon to be 'ex' can be done by obtaining an IRS Transcript. Both taxpayer and spouse are allowed to obtain this from the IRS.
To obtain is an IRS Transcript, click the link to get started. A transcript will show whether or not your tax return has posted in the IRS database.
To be clear, filing single is not an optional choice you get to make if you are not divorced on December 31, 2020. To review your filing status options use this link: What is my filing status?