Being in the zone is more acute and recognizable when one plays an outdoor sport for a moderate period of time. I used to play tennis competitively and I could clearly tell when I was in the zone. It would be amazing, I would feel like I could take on anything my opponent would throw at me and without even breaking a sweat. I would expend very little effort but still end up playing the best tennis of my life.

In programming, I doubt if it's as easily discernible. I've been programming for a few years and had a few productive stretches coding 8-10 hours at a time(with small breaks in between of course), but never felt like I was in the zone even once.