Dear 100 Hour Board,
How do you find the office hours for a professor with which you do not have a class?
Dear Mr. Ash,
Most department websites have a bio for each of the professors they employ. These generally include their office location, hours, and email. Just look him/her up.
The Soulful Ginger
Dear Professor John Smith,
If you know where their office is, their office hours are almost always posted by the door.
Email them asking if you can stop by during their office hours and ask when they are. That's what I do.