First, look for experts. This is reflected in years of experience and reputation. Successful person is always known to specialists and customers interested in translation. Ask your colleagues and friends about the best place that where you can find the best translator.
Second, send a sample of your project to a translation agency to be translated as a free sample. In this way, you will be able to verify the translation quality if you are familiar with this field; otherwise you can ask for help from someone who can help you (for example, your supervisor).
Third, you spend large amounts of money on your projects, so do not underestimate the cost of translation. Translation is a tiring and uneasy job, so do not underestimate efforts made by translators as your entire project may be lost in vain if you do not get correct translation of what you need. Translation is, to some extent, equivalent to the project as a whole. When the translator receives an appropriate rate, he/she produces high-quality translation.
Fourth, try to make clear every terms and names before starting translation in order to avoid correcting or re-translating the whole file.