TurboTax asks "Any foreign source qualified dividends or long term capital gains?" How can I compute that number from my form 1099-DIV?

That info is derived from the "Statement of Additional Information" you either received in the mail or downloaded from the issuer's website.

Often, if you had foreign source dividends, you would have an entry in box 7 and/or 8 on your form 1099-DIV, Foreign tax paid and Foreign country or US possession.