How do you tell when you're reading a poor translation?

John Ciardi, in his foreword to his translation of Dante's Divine Comedy, observes that any translation is, at best, a failure. He means it is impossible to convey the depth and levels of meaning of the original into another language. That point conceded, however, translations must be made and used. If there were texts in Finnish one needed to understand by next week, it is absurd to suppose that one should (or could) acquire mastery of that language in the time allotted. The fact is, there is important information in many languages that one may wish to know, and it is further absurd to suppose one can acquire anything like a thorough understanding of all those languages.

In many cases, a bad translation is obvious. In the English version of a flyer promoting a Japanese amusement-park ride, for example, park-goers were promised that they would be "brandished and inverted." In such cases the bad translation announces itself in strident tones, and one doesn't have to be John Ciardi to realize that a re-translation of the translation will be required in order to piece together the actual meaning.

But what about translations that do not set off grammatical or usage alarms, yet may still be poor renderings of the meaning of the original? Are there other, perhaps subtler clues you would look for that would make you question whether the translator has done the job well or poorly?

There are two ways that a translation might be bad: faithfulness to the original and fluency in the target language.

As you suppose, it is easy to detect flaws in fluency in the target language because they are apparent to anyone fluent in the target language. However, the only way to tell if a translation is not faithful to the original is to know both languages. In cases that you don't, you have to trust the professionalism and skill of the translator. If you don't know who the translator is or what his or her credentials are, then you have reason to be doubtful.

Any translation is at best an approximation, since it is rarely possible to convey the exact meaning of a source language text in the translated text. Speaking as a professional translator here.