You can convert to a traditional IRA pre-tax or after-tax. All the sites I've found assume you know how much of each happened. I've been contributing for decades, across several employers, rollovers, and accounts, so I'd like to verify my understanding of how much pre-tax and after-tax contribution there is in a particular account. How do I do that?
The post-tax amount in the Traditional IRAs (officially called the "basis" of the Traditional IRA) is simply the total of your Traditional IRA contributions over the years that were not deducted. (You needed to have filed Form 8606 Part I with your tax return for every year that you make a non-deductible Traditional IRA contribution, but if you forgot to file it for a given year, you can go back and file it now.)
Everything else in Traditional IRA (including Traditional IRA contributions that were deducted, as well as all earnings) are pre-tax. In other words, the post-tax amount does not change as your Traditional IRA investments grow naturally, so as time goes on, an increasing fraction of your Traditional IRA will be pre-tax.
Employers have very little to do with IRAs except insofar as you need to have had an employer (possibly yourself) because contributions to IRAs require the IRA owner to have had compensation (earned income such as salaries or wages, self-employment income, commissions on sales etc). Are you thinking of 401(k)s (which are employer sponsored plans) that you have rolled over into IRAs in earlier days? As a general rule, most 401(k) plans have some pretax money even if you have contributed only to Roth 401(k)s. This is because the employer match must be deposited into a pretax account, not into the Roth account , and when you leave service and do a rollover into an IRA, the Roth portion goes into a Roth IRA and the pretax portion (from the employer match and your own pretax contributions) goes into a Traditional IRA.
This site says
Both Traditional IRAs and Roth IRAs have an IRA basis that must be tracked on IRS Form 8606.
IRS basis is the money in an IRA (whether Traditional or Roth) that has already been taxed or was non-deductible.
Investors and/or their tax professionals are responsible for keeping track of the IRA basis.
So I think form 8606 will have the post-tax amount of all my traditional IRAs combined.