Currently there is no way to translate a piece of text and see which source language you identified it as in one request.

You first have to make a request with the text to /Detect to get the language, then a request to /Translate to actually translate the text. This adds unnecesarry overhead on your systems because the /Translate call must identify the source language of the text again. It also doubles the cost to use your translator, since a 10 character sentence now would cost 20 characters to translate (due to the duplicate request), and it would take twice as long as normal.

I suggest you simply add the source language as a parameter of the /Translate response, alongside the translated text.