Taxable income, line 10 of form 1040.

The actual threshold is $78,950.

Using an example: your line 10 is $80,000 and that includes $10,000 long term capital gain (LTCG).  $1050 (80,000-78950) of your LTCG will be taxed at 15% and $8950 will be taxed at 0%.

Turbotax will prepare the "Qualified Dividends and capital gains work sheet" to show the tax calculation. In some cases, a Schedule D worksheet will be used, instead.