I have a question re all-wheel drive in a 2001 Subaru Legacy.?

I have a 2001 Subaru Legacy Sedan. I was under the impression that the all-wheel drive kicked in automatically. Regardless, I never 'pushed a on' or did anything else, and yet the car has been notably stable on dangerous winter roads since I got it. Two days ago, I became stuck in a snowy yard that was a lot less than my car has been able to easily take before. In getting me unstuck, my brother said it did not appear that my all-wheel drive was working, and we searched in vain for a switch or on that might engage it. I have lost my owner's manual. Before I take my car into the local garage and run up a bill (or look like an idiot because I don't know anything, lol), does anyone know if there is supposed to be a on or switch for the all-wheel drive, or is it an automatic thing, and therefore I need to go to the garage to have this checked out? Thanks!