The short answer
A first-principles test for deciding whether intelligence improves the interaction or merely decorates the pitch.
Name the job
AI earns its place when it performs a defined job that a simpler system cannot perform well enough, such as perception, generation, adaptation, ranking or summarisation.
If deterministic rules work just as well, the simpler architecture may be faster, cheaper and easier to validate.
Evaluate the whole system
Model quality is only one variable. Latency, operating cost, privacy, comprehension, fallback and human review determine whether the experience is dependable.
A strong model can still produce a weak interaction if it is slow or incomprehensible.
Prototype the uncertainty
The first prototype should test the highest-risk assumption, not imitate a finished product. Use representative inputs, acceptance criteria and failure examples.
The objective is evidence for a decision: continue, narrow, redesign or stop.