How to tell if my smoke detector is hardwired or battery powered?
If it is hardwired then there will be a cable going to each smoke detector. It is usually pretty obvious. If you don't see anything then see if you can remove the smoke detector from the ceiling. If you can remove it easily (typically by twisting it slightly and then pulling away from screws and/or a mounting bracket) and it comes off (except possibly for a small mounting bracket) then it is battery powered. If you try to remove it and find it is hanging by a cable the it is hardwired.

If all that doesn't make sense, post a picture of it and we'll see what we can figure out.