If you only see it through the viewfinder then it's probably on the mirror or in the viewfinder. These cannot show up in photos as the mirro and the viewfinder are not in the light path when the photo is taken.
Small hairs and dust inside, or even on the lens, usually will not show up in photos. Think about it, you lens has a minimum focusing distance of say 2' maybe, and where is that hair? It's at 0'; the lens cannot focus on it even if you tried. You can use this to your advantage when shooting through wire fence or glass with wire in it. Put the lens right up against the wire, and even though it is going across the lens, it will not show up in the image. The lens can't focus on it.
If the hair/dust is at the edges of the lens, then that part of the lens isn't even used unless you have the aperture wide open.
The worst I've even seen is large bits of black paint flecks scattered all through out the inside of a lens, and even that didn't show up in the photos, although it did cause a lowering of contrast in small areas of the image.