Because the basis in nondeductible traditional IRA contributions carries forward from one Form 8606 to the next one required to be filed, if all Forms 8606 were correctly filed your basis in nondeductible traditional IRA contributions carried into 2023 would be the amount on line 14 of your most recently filed Form 8606 for some year prior to 2023.
When examining old tax returns, they would normally be collated with the pages in the order of the sequence number in the top right of page 1 of each form or schedule. Form 8606 has sequence number 48, so it should be present after lower sequence numbers and before higher sequence numbers if the pages are ordered properly. If you have pdf files of your past tax returns, search them for "Sequence No. 48" to find the Form 8606 quickly. You can also search for "Form 8606" but this might match in several places in your tax return. Another option is to search for "your total basis in traditional IRAs" which is present on line 14 of Form 8606.
