Dear N___:

You wrote:

>1-I went to a place where I used to work.
>
> Can we tell from this sentence whether I was still working there when I went there?
>
Of course. “Used to” means that it is something in the past. In a sense, your time there is all “used.” Whenever you “used to do” something, it means that you no longer do it. That is what it means.
>
> Can one say:
> 2-I went to a place where I had used to work.

No. This makes no sense in English. “Used to” in the past sense does not take any auxiliaries. Try saying “I went to the place where I had worked” if you think you must put it in the past perfect.