The proof of the pudding is in the eating

Some days I feel curmudgeonly. And here's why.

How does a client know if a translator is good? Experience, education, qualifications... If you want to hire a translator, should you require that he or she has a degree in translation, a degree in the subject matter, sufficient experience...?

Here's a query about how to translate: "Do not make change for a customer outside of their order."

A difficult sentence? Tricky? Requires specialized technical subject matter knowledge?

Two translators with university degrees in translation thought it meant "Do not make any change that the customer has not ordered," while another translator, with over 20 years experience, thought it meant "Do not make changes to the customer's order." And yes, all of them purport to translate professionally from English.

So if you were going to hire a translator, would you judge them by their qualifications and experience, or by their product? Why are translators so resistant to doing tests? On various translator forums where translators voice their opinions, I read that tests are useless; tests only take advantage of translators and yield no benefit; that a potential client can better judge the translator's competence from their qualifications, education and experience.

On the contrary, I see evidence time and again that the proof of the pudding is in the eating, not in the recipe.