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?