how do you verify the effectiveness of a can?

Unless you have it tested by a lab, I do not think you can. Every time I used mine was testimony of its effectiveness. Once, I had a bottle of this same brand for over two decades and it worked when I needed it. It is discharged in a stream pattern and it is harder to use on smaller quick moving dogs. A mist might be better. Remember that using any product, such as bear spray on a dog, on anything other than its "listed" purpose may get you in a heap of trouble because of the Federal use laws.

what are the ingredients?

This is what it says on the can: active ingredient: capsaicin and capsaicinoids 0.35% derived from oleoresin and capsicum. Other ingredients: 99.65% contains petroleum distillate.

is it banned in any State?

I googled your question and my sense is that using pepper sprays on people is where you can get into trouble but that sprays made for use on threatening/attacking bears and dogs are not banned in the 48 contiguous states. I got the impression that sprays for use on animals are not as strong although I am no expert. http://www.ebay.com/gds/States-With-Pepper-Spray-Restrictions-/10000000032578507/g.html

Would this work on chickens? Don't laugh. I'm serious. They attack my cat.

Not laughing. Surprised cat didn't have chicken dinner. Be honest don't think I try it. I think might cause more harm. Might try water in spray bottle. It's safer less chance harming chickens eyes.