Some stud finders will work in that situation. You could try small head screws (sometimes called trim screws). These would likely go through the carpeting and not show. When doing this, be especially careful not to catch a thread in the carpet or you'll pull a chunk out!
I've seen one company that makes screws with break away heads for this purpose. If the subfloor is not very thick, they probably won't hold well. Check this out, it may work for you. http://www.squeaknomore.com/
It's much easier if you have access from below - a bunch of companies make brackets to pull the subfloor down to the studs.
Some finish nails toe-nailed might work, too but could be a little tricky. SOmebody got some other ideas?