I'm trying to think how I would reliably prove flame in a way that would resistant to annoying false negatives in the face of a very low simmer. (Somewhat ironically, this is probably the most important setting for which you'd need it. I'm not in any real danger of having a full-bore gas burner go out. I am in danger of having the lowest possible simmer blow out undetected.)

Over a long enough period of time, a sufficiently sensitive raw temperature sensor is probably good enough, but that's not likely to be cheap nor reliable/long-lasting.