The real answer to "How do I judge" is to learn Greek or Hebrew yourself and look at the translation. Or, I suppose you could move to Turkey and pick up some Greek in the places where the Bible was actually written, but you'd have to be crazy to do that :)
For the non-language learner, a Greek or Hebrew Interlinear will give the root word for each translated (English) word in the scripture. Ideally, one would understand the part of speech, tense, mood, position, etc... and compare it with a corpus of literature from the same time period like Persues to see how a given word was used in context at the time. Tools such as BibleWorks or Logos (http://www.logos.com/) will aid tremendously in this regards. (A seminary degree doesn't hurt here either!)
For the rest of us, we need to rely on the work of scholars. There is a great deal of scholarship that goes into a good bible translation, and like any academic pursuit, it should be judged on how well it follows the rules of good scholarship.
First off, one should consider the manuscript evidence on which a given translation was based. Many older translations, such as the KJV, were limited to the Textus Receptus, and based on fewer available manuscripts. Since the 1533 printing of this version, for example, Codex Sinaiticus and the Dead Sea Scrolls have simply added more source material that help scholars detect variations between various handwritten copies. (That said, no major doctrine has ever been built on one of these variations)
Secondly, one should consider the purpose of the translation. Some manuscripts, such as the New Living Translation will purposely diverge from a word-for-word translation, because that is the aim of the producers. In contrast, the Amplified Bible is all about giving as many synonyms for a given word as possible, intentionally undermining its "readability."
Thirdly, one should consider the credentials of the translator. The late Bruce Metzger is probably the foremost authority in this regard. His critical apparatus A Textual Commentary on the NT identifies all manuscript discrepancies and grades them on a scale of A to C to distinguish the likilhood of a given reading being an original. It should be noted again, that in most cases, a variant is no more than a letter or a single word.
In the end, judging the "correctness" of a translation is an art, not a science, but the same tools mentioned above will help.
Luckily for us, there is very little in Scripture that is rooted in the language. Unlike the Qu'ran, Christians do not hold Hebrew, Greek, Aramaic, Latin, or English in any particular high regard. For a religion that spread around the world, mostly amongst the poor, it would have been odd to have gotten hung up on language.
In the final analysis, if a doctrine is rooted in only one language, it is unlikely to widespread or even important. As Mark Twain once said, "It's not the parts of the Bible that I don't understand that concern me - it's the parts that I do understand that terrify me."