I would like to propose a few things to look for when selecting a translation provider.
Location relative to language
As an extremely near-fetched example, if you need a translation from or to a Nordic language, your best supply of skilful translators is likely found in or near a Nordic country.
Would you be satisfied with the quality of your translation if it equated that of the provider's own web pages? The same test can also be applied to correspondence, such as email.
Ask how the translator who would handle your assignment has demonstrated his or her subject matter competence in the domain of your text.
You may not find a brain surgeon to translate your text on neurosurgery. However, in such a case, your translator should at least be familiar with health care.
Translating a text is not always the best way to get your message across. Cultural, legislative or other differences may cause a situation in which the best plan of action is rewriting the text more or less from scratch.
Your language service provider should be able to recognize and handle such a challenge.