For me the best test is when I've already correctly diagnosed a problem with a watch, and then bring it in for an estimate. If the estimate includes a bunch of superflous stuff to simply run up a bill, you have your answer. A simple example would be breaking an acrylic crystal on a watch that has just recently been serviced. Assuming that close inspection reveals an unharmed movement keeping good time, the crystal change should be a relatively inexpensive proposition. Some places will tell you that you must have a full service or they won't touch it--thanks, no thanks.
Do beware that most "brand authorized" watchmakers may be required to be a bit trigger happy with replacement parts. Some of that is dictated by Rolex, Omega, etc. (That's also why I avoid these outfits unless I have a specific reason for needing their services that cannot be obtained elsewhere).
Depending on how much time you and your potential watchmaker has, I've had some amazing experiences being invited back to the bench for an afternoon. Genuine curiosity and friendliness can open many doors.
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