There is not such a thing as “in ride identification of FTP”. An FTP test is done with a certain reason following a certain protocol. Only those values with the same protocol can be compared with eachother.
Furtermore, you don’t do an FTP test that often. Some trainers may say once every six weeks, but most of us do it max a few times per year. Also and not quite unimportant, you need to be free of stress in your muscles of any effort you did in the days prior to an FTP test.
Don’t put too much focus on FTP, it is just a number to set your training zones. It may vary easily like +5 of -5 just because of how you feel that day. It is not that important. Take it on an higher view and take a look of your year-by-year progress or half-year. What’s in between is not that important.
For most cyclists there is more than Zwift alone and Zwift only keeps track of your rides within Zwift. Just looking at the numbers (in the winter period) within Zwift is a very limited view of all of your data.
The main cause of this discussion is that Zwift makes FTP very important in their category system to distinguish the various levels of riders. And they implemented this “Hurray, an increase of your FTP is detected” screen which is quite funny for me to see.
However, I agree with John Poole some measurements can be used to validate efforts by riders if the values for power actually match their previous efforts. But then you should still be very carefull since you never know the context of a certain ride.