Most of those tests (almost all since the Limited Test Ban Treaty of 1963) were underground, where no one could witness them except in photographs. Since the LTBT over 60 years ago, China has done 22 above-ground tests, and France did 50. France stopped atmospheric testing in 1974 (over 50 years ago), China in 1980 (45 years ago).
So the last test at which it is feasible that someone watched with their own eyes would be the Chinese nuclear test at Lop Nor on 16 October 1980, at 04:30:29.67 UTC.
That was a test of the JL-1 missile, which was fired off and then flew all the way through reentry to nuclear detonation. At the time, a lot of people in the West doubted that Chinese missiles were capable of delivering a working warhead (1) so they proved their capability by firing an actually armed SLBM and it detonated in the atmosphere, exactly like it would do against a city.
1: Much like a lot of people discount North Korea's missile delivery today. No one doubts that Israel, France, Russia or the UK have missiles that can deliver nuclear warheads successfully (the US, like China, has actually done a full SLBM launch-to-warhead detonation test, the only two countries to do so- though the US Operation Frigate Bird was before the LTBT). I wonder what it is that connects China and North Korea, and makes Westerners doubt their abilities in ways that they don't doubt the French or the UK?
So the last test at which it is feasible that someone watched with their own eyes would be the Chinese nuclear test at Lop Nor on 16 October 1980, at 04:30:29.67 UTC.
That was a test of the JL-1 missile, which was fired off and then flew all the way through reentry to nuclear detonation. At the time, a lot of people in the West doubted that Chinese missiles were capable of delivering a working warhead (1) so they proved their capability by firing an actually armed SLBM and it detonated in the atmosphere, exactly like it would do against a city.
1: Much like a lot of people discount North Korea's missile delivery today. No one doubts that Israel, France, Russia or the UK have missiles that can deliver nuclear warheads successfully (the US, like China, has actually done a full SLBM launch-to-warhead detonation test, the only two countries to do so- though the US Operation Frigate Bird was before the LTBT). I wonder what it is that connects China and North Korea, and makes Westerners doubt their abilities in ways that they don't doubt the French or the UK?