I think strong double vision is always a pretty good sign of getting into the "too much kava" zone. It doesn't have to be unpleasant, but if you keep drinking you might get strong nausea.
The worst "too much noble kava" episode I've ever had was in Vanuatu. My first night there, my first nakamal crawl, after a few shells (beautiful) I went back to my hostel. A random guy brought a few plastic bottles from a nearby nakamal and insisted that we have a few extra shells. I had two or three and the world started spinning. I could barely walk. I quietly crawled back to my room, every noise, every bit of light seemed offensive. I had to lie down and prayed not to vomit. My mind was alert and my thoughts were clear, but my muscles, my eyes and my ears were clearly very confused. I finally managed to fall asleep. I woke up totally dehydrated, but strangely without any big hangover. But I couldn't even think about kava.