In this short clip I’ll explain you how to calculate the true airspeed. Knowing the true airspeed and the ground speed, we can determinate whether we have headwind, or tailwind.
It’s very simple:
FL 75 (7.500 ft)
IAS = 90 kts
TAS = 100 kts (TAS inceases with increasing altitude)
GS = 109 kts
if
GS > TAS => have tailwind
GS < TAS => headwind
This means in our case, we have 9 kts TW (tailwind).
Fly safe! 😀
Fly with Stan - Sharing (not only my) flying experiences... 
Worthwile to mention and in addition to the above:
Reading out indicators in cockpit showing that TAS increases – GS decreasing and leveling out around 0 kts combined with rapidly decrease in FL is nog indication for TW or HW but you might consider transmitting Mayday.
C. Aptain
Hahaha, I was trying to imagine this scenario… But: Following your theory, if the GS would decrease to 0 kts, it would mean you have 100 kts headwind and would be just “hanging in the air” like bird. No need to panic and to declare an emergency, only you are not going to move nor forward, nor afterwards.
Maybe after a couple of hours when you run out of fuel, and your engine quits, you would have to declare an emergency… 🤣🤣🤣🤣🤣🤣
Hmmm, correct me if I’m wrong but TAS is given by a read-out coming from the ASI calculated through differences between static and dynamic pressure. If you would ‘go down’ vertically, head towards the ground, TAS would increase, FL decrease and GS will level out around 0 kts. One could agree that GS will eventually drop – pun intended – down to 0 kts. No hanging in the air …
Mmm 🤔. The difference between the dynamic and static pressure will give you the IAS, which you’d be reading directly from your AI (airspeed indicator) 😉. The TAS will be increasing by 2% for each 1.000 ft…
If you start descent as a kamikaze; indeed the IAS en TAS will be increasing and the final result when reaching FL0 would be a GS of 0 kts; and most probably a crash… 🤦🏻♂️😂😂