There is a scent, I believe it is a hormone but I don’t think it is what the body actually produces. If a dog reacts to it they can try to train it. If it doesn’t, they train the dog for something else or find it a good home. They also have people fake seizures. You can probably find some YouTube videos of it.

We have a dog that could do it. She could warn us about my partner’s seizures and fainting a few minutes before it happened. She could fetch help and apply pressure to the abdomen. (I say could because my partner had surgery a year ago and hasn’t had any seizures or fainting since then.)

I definitely noticed a change in my partner before seizures (but not fainting). The dog alerted on me once during a moment of high anxiety. I wouldn’t begin to argue WHAT the dog is actually reacting to, but the training methods people are using seem to work.