In TurboTax Online, when you ask to try sales tax deduction anyway, at the final screen of that series TurboTax tells you which method was chosen because it provided the higher deduction, then provides a radio button labeled "Change your deduction" if you want to go back to the other method. See screenshot below:

See this tax tips article and this help article for more information.
If both the sales tax method and the state and local tax method produce a result over $10,000, it won't make a difference on your Federal return, as these deductions are capped at $10,000 under current law.
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