It would be interesting to see how toxic these newts would remain if the garter snakes were eradicated. If this was indeed a costly trait, we should see a drop in toxicity over a long period of time (possibly evolutionary time). To rule out coincidence, you could follow multiple lineages as they speciated.

In fact, looking at related newts whose ancestors were toxic (assuming the trait is not novel in these ones) would give us some idea as well.