The ＃1 way to tell a good translation
To tell a good translation from a bad one, ask yourself this: Does it read naturally?
The road to translation hell is paved with slavish adherence to the source text
Some translators follow a word-for-word approach in which every word of the original text has a corresponding word in the translation: if there are six words in the original text, there must be six words in the translation. In less extreme, but no less inane, situations, a translator will use a word from a dictionary that results in an unnatural, and even an unintelligible, translation. When the translator is asked why they wrote this horrible sentence, the response is often, “that’s what it says in the source text!”
Of course, the translation must reflect the meaning of the original. However, you can do this and write well. This reminds me of George Orwell’s famous sixth rule for writing well: “vi. Break any of these rules sooner than say anything outright barbarous.” (Politics and the English Language)
The Holy Grail of Professional Translation
In contrast, the holy grail of most professional translation is a translation that reads like a document written in the target language by an educated native speaker. That’s a mouthful, but it means that the translation should read naturally. The reader should not be able to tell that it is a translation. Creating such a natural translation is easier said than done, and I could go on at great length about the whys and wherefores, and yes there are other issues, but all other things being equal, if the translation reads naturally, then the translator did a great job, and this can be your measure of quality even if you do not know the original language.