Normally, it takes two indicators. One for the travel along the carriage, and one for the cross slide. If you use the cross feed dial, you can use a test indicator to find a zero a ways back in the spindle bore, zero the travel indicator on the carriage, note the dial reading on the cross slide. Move out an inch, dial out until the test indicator is zero again and note the difference in the cross slide dial. That will give you the taper per inch directly. Or per side, depending on the machine.
Same principle on an OD taper.