The vast majority of orbs are dust, when photos are taken inside. You can also get lens flare or light bounce off, especially if you have glass coffee tables or mirrors in the room.
An orb has more likelihood of being "ghostly" or of a paranormal nature when it indicates movement--it's blurry while all the photo elements around it are clear--and has a "tail" like a comet behind it. It also really helps if, at the same time, you are video taping or audi taping in some way and get an EVP (electronic voice phenomenon). As far as evidence of something paranormal, orbs are pretty far down on the list.
Is there any other activity going on that would make you start taking pictures?
You might speak to any possible spirits and ask them to show themselves while you are photographing. Explain what it is you're looking for!
Good luck!
Ghostie1