if there was an AMT tax carryforward it should have shown on form 8801 on line 26 (2020 vers of 8801) the form would have been generated each year there was an AMT tax credit carryforward.  however, if you did not claim the amt credit properly on prior returns, it's too late to amend them so the credit would be lost.  you don't get to pick the year to use the c/f.  2013 is carried to 2014 and if any is left carries to 2015 and so on.  

This makes sense and thank you for the explanation, however it seems like there must be a way to prove that the credit was earned given that there is good record of having paid on old 8801 forms and subsequent tax records. Is there precedent for this?