How is true airspeed measured?
You don't measure TAS. You measure IAS (or CAS), then correct for altitude and nonstandard temperature to get TAS. As a rough rule of thumb, add 2% to the IAS per 1000ft of density altitude.
With modern avionics, you know your GS from GPS or similar systems, so the difference between TAS and GS tells you the head/tailwind component and course vs heading tells you the crosswind component, which will be combined to display the direction and speed of the actual (not forecast) wind.
With older (or no) avionics, you get a winds aloft forecast before departing, and you do the reverse of those same calculations with an E6B to find out what heading to fly to get the desired course.