You would not have any Forms 8606 showing an adjustment of basis if you have had no reason to file Form 8606 Part I (for a new nondeductible contribution made to a traditional IRA or for a distribution or Roth conversion from your IRAs).
Determining your after tax basis would have to come from you records, either from box 5 of Forms 1099-R as previously mentioned or from statements from your 401(k)s.
