I'm living on a steep hill, in the basement room of a house with
several other people. I just moved in a couple days ago. Yesterday
morning, my bathroom was having drainage problems - the shower and
sink drains were both running MUCH slower than normal, and, when I
flushed the (empty) toilet, it nearly overflowed, as if it were
clogged with something.

This isn't a permanent problem - when I came home that evening,
everything was back to normal.

Now I'm wondering if that means that, when my drains were running
slowly, someone upstream of me in the plumbing system was running
water, and it was passing through my pipes. This concerns me, because
I've heard that, sometimes, when you're at the bottom of a big
plumbing system, if you have plumbing problems that prevent waste
water from upstairs from going into the sewer, the waste water may
instead come flowing out of your toilet, drains, etc. I certainly
don't want the junk from other peoples' toilets coming up in mine.

I have two questions: How do I decide if I'm at risk for this kind of
backflow, and, if I am at risk, what can be done to prevent it?

I once lived in a basement apartment that had such severe plumbing
problems that sewage waste sometimes bubbled up into my bathtub from
the drain. My solution to the problem was to find another apartment. I
doubt that there is anything a tenant can do to prevent this kind of
thing.

You may be getting backflow for other reasons. For example, if the
pipes are old and full or corrosion.

Kriswrite

You may want to post at www.plbg.com they've been a big help with my
plumbing questions,there are some master plumbers answering posts
there.