Professional translation agency should not be responding to orders for a rate below a certain threshold. If you would agree to a price of 5 dollars per page, there is a high possibility that the translator is inexperienced. It is very easy to find an offer of cheap translations, but typically this offers are given by philology students looking for their first job. In this case, it is difficult to expect high quality translations.
You should always check whether the translator or a translation agency runs a legitimate business. One simple way to check it a visit in the Central Statistical Office. If an interpreter is not active, there is a high chance that we deal with a student or a young translator without the experience.
A professional translator should have a website, preferably on his own domain. On the basis of the quality of text on the website, we can figure out what kind of quality of translations we can expect from him.
Professional translation agency should use CAT tools (computer-aided translation). These are computer programs which support the translation process. CAT tools do not perform work of an interpreter, but provide him with some help, and accelerate the translation process. They are important when we talk about a long text, or if we're going to sign a contract with a translation agency for a long time and order numerous translations. Translation memory programs store previously translated source texts and their equivalent target texts in a database and retrieve related segments during the translation of new texts. Using CAT tools mentioned above doesn’t tell who is a professional translator and who is not. However, it can be assumed that the good translation agency should be using not only CAT tools but also Translation Management Systems such as XTRF.
When you're not sure of the proficiency of a translator, you give him only a piece of the whole work, just for example, the first chapter. On the basis of cooperation on this chapter, you can tell if it is the person you want to cooperate with.