No assumption other than all taxable. Any non-deductible basis would be captured on form 8606. You can request tax transcripts to see if form 8606 was ever filed. They may have even filed it annually just to carry the basis forward. You can locate contribution dates and the tax returns for the corresponding years. You may be able to prove the non-deductibility of some.
With no basis proven, it will all be taxable.
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