Well, the best you can do is ball-park it. Estimate how much of each food you eat, then go home and plug that estimate into
FitDay - Free Weight Loss and Diet Journal or something of the like.
There is nothing else you can do. If you don't prepare it, how can you be certain?
You can't.
When I was in college, I ate at the caf sometimes. However, I also grocery shopped and prepared most of my meals each day. And that was with 2 majors and a job!
It is a personal issue though. I wanted to be in complete control of my diet b/c I had serious aspirations in terms of physique transformation. If you are merely trying to lose weight, I am sure you can wing it and be fine. Just control your potion size.