Sprocket wear indicator?

I haven't had it for very long, but I really love my Park Tool chain wear indicator. I like gadgets, and this one is cheap, easy to use, and tells me in no uncertain terms when to replace my chain, which I would otherwise postpone indefinitely.

But sometimes I'll replace the chain, figure I'll try to use it on the old cassette, start using another bike and forget about it. Or "something" will start skipping and being annoying, I can't quite figure out what it is, but it seems like one of the smallest sprockets is worn out, but the chain seems fine. And again, can't remember when I replaced it.

Anyhoo - I would like a sprocket wear indicator. I've only found one, a Rohloff, and it surprises me a little that Park Tool or Shimano haven't made one. Anyone use one? Are they useful or should I just go by the "wave-pattern" and skipping behaviour?

I think that even if you had a sprocket wear gage, it would only be an indicator of whether a new chain might skip or not. You'd still have to put on the new chain and ride the bike to be sure. It may depend on the brand and exact shape of the links in the chain compared to the sprocket.