I live in Germany, and I've developed a similar rule with doctors in general. But here I can use an even better rule of thumb - be extremely suspicious of anything that isn't covered by public health insurance. I'm sure other people have had different experiences, but I'm 40 and so far almost every time I had to pay out of pocket it turned out to be some controversial or even pseudoscientific BS. The last one was some weird back pain therapy - I wasted my time and money on that until I read some paper on it. Its conclusion was actually that the therapy "probably works", but when I actually read it, the data ranged from "inconclusive" to "not effective". And that was the paper the company selling machines for that therapy was linking to.

What worked in the end was - surprise - lifestyle changes.