I think it's basically when withdrawal symptoms become progressively more acute after each detox - I'm sure a google search will produce a tidier answer than that though
I've certainly experienced it first hand. I used to be able to drink daily for weeks and quit afterwards with little more than feeling a bit irritable - the last time I drank however, I did so for two weeks and when I tried to quit I had shakes, mild hallucinations, panic attacks etc etc. The doctor warned me that if I started drinking again and had another detox, I could potentially have a seizure and the time after that could progress into full blown DTs! Scary stuff!
If you ever feel fearful or anxious after a binge and/or have any other physical symptom that May alarm you then get to a DR straight away. The quicker you can detox safely, the less likely you'll experience kindling in the future.