But you're also putting faith in the person who prepared it that it truly is decaf.

I've been burned by getting regular coffees at restaurants and coffee machines when I ordered decaf.

I wonder how hard it would be to have a "litmus paper" style test for caffeine.

About 15-20% of full strength. Hardly negligible.

More like 5%, but it's high enough that if you measure your coffee intake in pots/day then there is still enough caffeine for physical effects and dependence.