How to tell which induction cookware is most efficient in conducting heat?

Your rule of thumb for induction cookware of any sort is that if a magnet will stick to it, it will work. If you can get to a store that has the kettles you are interested in, bring a magnet with you to test the various models. The better the magnet sticks, the better off you probably are.

I would give a couple of extra things some consideration. The heavier the pot, the more consistent the heat, though most induction cooktops are really good at maintaining a temperature. Second, go with stainless if at all possible. It's so much easier to clean and maintain.

Edit:

Just climbed out of another internet rabbit hole. Induction cookware works because of a current induced into metal that does not conduct electricity well. Iron works great because it is not a totally efficient conductor. Stainless steel, because of the different alloys, does not always work on an induction cooktop. Clad pots and pans that have layers of ferric metals sandwiched between layers of better conductive metals can give you a balance of heating ability and conductivity. All of this, though, can in general be boiled down to: Heavy is good, Magnetic is Good, Stainless is Good. You can check these in store before you buy.

If you are looking to buy online, you face a different set of problems. An "Induction Ready" label is a place to start.

Second Edit:

Fantastic Information here. Tells you a lot more about how the whole shebang works, so you can decide what you will require in a Kettle.