Your agency can also help in these situations. As Steve has said, you can contact them near the end of your assignment. If you don't know what that is supposed to be, someone at the agency probably does. I would contact my rep at the temp agency and ask them to start looking for a new job for me. The rep would contact HR at the company I was working at, HR would contact my supervisor, and the rep would learn if they wanted to extend my position or end it.
I found that frequently direct supervisors did not want to tell people they were being let go (it's not the easiest conversation to have, even with a temp), but had no problem telling HR the truth. It's a round-about way of getting the info, but it is info that you really need in order to plan your life.