How much of a bond ETF's dividends are tax-free for US non-resident aliens?

Normally, for US non-resident aliens, the withholding tax on dividends is 30%. However, dividends that qualify as "portfolio interest" are not taxed (i.e 0% tax). When a US domiciled bond ETF pays out dividends, part of the dividend may be classified as "portfolio interest", so the total dividend withholding tax paid may be less than 30% of the dividends. The question is: how could a non-resident alien know the proportion of dividends that is tax-free before buying a bond ETF?