First off....you are not going to be allowed to do anything with regards to patients.
Secondly, just ask questions. When I went in for my observations I met the PT before we started and asked them how the observation would work. Would I be allowed to ask questions during the session, etc... On one of my observations I was told patient background before we went into the hospital room. Once in the room, I was to just "stay out of the way" basically. At my other two observations, I was allowed to ask questions during the treatment.
I don't know how everyone else felt, but my observations were incredibly boring due to the fact that I was not able to actually do anything. It was helpful in that I got to observe the work environment, patient/PT interactions and paperwork involved. It doesn't take very long to take all that in, then the rest was just following the PT around. During break times you can ask what the hours are like, job availability, etc.... Again, that takes very little time.
For me, the purpose of the observations was to check out the environment and verify that I did indeed want to work in that environment. It was not a "show me how to do PT in a couple of days" exercise. I only had to have 45 hours and I was thankful it wasn't any more than that. I love working and I will like working as a PT, but I abhor just watching other people work.