I suggest making a spreadsheet (excel/google/whatever). That way you can keep track of hours and the specialty hours, such as neuro, ortho, geriatric, pediatric, etc. Those divisions are required when you finally submit your observation hours.
It's something I hadn't taken into account when I was first volunteering. Luckily I kept notes on what I observed, and from the notes, I was able to make divisions between the types of sessions. Plus, I think it's nice to be able to look back and understand more about some treatment you observed--just from having more experience or classes.