How do I find out whether simplified AMT FTC limitation was ever elected?

If you had previously elected this, you would only have one Form 1116 for each type of income.  If you had not elected it, you would have 2 Forms 1116 for each type of income, one for regular tax and one for AMT tax.

How do I change my selection whether to use simplified AMT in TurboTax Online?

It's confusing that you write: "you would have 2 Forms 1116 for each type of income, one for regular tax and one for AMT tax." The second half of the sentence makes it seem like there's one form for each type, but I think you mean: "you would have 2 Forms 1116 for each type of income -- that is, four forms total, two for regular tax and two for AMT tax." Unless what you meant to say was "you would have *one* Form 1116 for each type of income, one for regular tax and one for AMT tax -- two forms total".