A FEW SUGGESTIONS
(How to test translation suppliers using sample files)

   Send at least 2 test documents – a simple one and a difficult one.
Do they handle difficult stuff as perfectly as with easy ones? Are they able to offer professional service in technical field? Many people can do simple translation nowadays, but quite often they perform awkwardly in translating difficult text.(e.g., to our experience, Legal / contract text is a nightmare for many “average” translators. )

   See how fast they return a test translation.
If a translation agency returns its test translation for a 300-word sample document more than 24 hours after you have sent it to them, it’s quite likely they are too busy to handle your regular assignments in a timely manner.

   Send your test documents in a complex format/layout.
Do they return a completed file whose format can exactly “mirror” the original? If you give them a PDF, they should return a PDF in the same layout. If the original document includes a table, there should be a same table in the translation document.

   Check the details.
Do they pay attention to the details (such as punctuations and pagination) of their translation outputs? If they don’t even know the difference between a Chinese quotation marks and an English quotation marks, you won’t expect them to deliver a translation that best suits the readers in the target culture. 

We believe this way you would easily and quickly screen out your desired suppliers!