Thrash is what happens the moment you run past your brainwidth: you stop doing work and start only switching between work.
The honest tell is reading. You get through four pages of a book, turn them, and retain nothing. Your eyes ran. Nothing loaded. The motion was real; the throughput was zero. An operating system fails the same way, by the same name: so much swapping between tasks that the machine spends all its effort moving things in and out and none of it running them.
It looks like work, which is what makes it dangerous. A thrashing day feels busy and produces nothing, and you usually cannot tell from the inside. The fix is never to try harder. It is fewer open threads.