What's the best way to determine this so I can input accurate info into the software?

You answered your own question right here:

To determine the portion of Box 1a that is foreign dividends, look at the paperwork provided by the dividend payer. It should list a foreign amount or a foreign source percentage you can use to calculate the portion that is from a foreign country.

Check your account statements. Look at each dividend payment. The foreign tax withholding will almost always be stated separately from the dividend, making the job easy.

BTW - don't you know enough about your investments to know which companies are foreign? Or which mutual funds invest in foreign stocks?

--Peter