1) Always, and I mean always, look for a translator that is a native speaker of the target language (to which you want to translate). For example, if you want an English-Portuguese translation, choose a translator from Brazil or Portugal. Many translators speak a language fluently but that is not enough to assure a good translation. This mistake is common in Brazil and I am always shocked at the poor quality of translations in Brazilian state institutions, like Banco do Brasil (where I once saw a sign that read, Foreigner With Drawl, which I assumed was supposed to read, International Withdrawal) and Infraero (too many to count, I'm afraid).
2) Never base your final decision on price. I know people who have hired "cheap" translators to reduce costs and have gone straight ahead and published entire books, without even bothering to hire a proofreader, only to discover that the text was crawling with errors and misinterpretations. If you really must hire a cheap one, ask for a CV, sample texts or ask someone who knows that language to test the translator (see our
services) before you close a deal.
3) Hire a proofreader or ask a reliable professional to review the work before publishing or sending.
4) Check areas the translator knows well, especially in the case of technical translations. Some areas are tricky so you might have to provide reference material or examples of similar texts.
5) Never be afraid to ask for references. Most translators have worked on websites or some published work you can look at.