How can the state of consciousness of a worm be evidence for anything, if you can't observe the worm's state of consciousness?

You can infer worm's state of consciousness by observing its behavior, like for humans.

By observing it's behaviour you can infer that in some sense it experiences the world. Experiencing events isn't the same as being conscious. If I strike a cricket ball, it experiences the blow; but it isn't conscious.

It's how we establish existence of consciousness in humans and other animals. What you said doesn't differentiate between worms and humans.

First prove that you have consciousness, then we can talk.

How is it neasured

No need because some one just said that consciousness is not a thing that can be measured, HA HA.